Nya is Kai's younger sister. She was the only family he had when their parents disappeared while they were young. Because of this, Kai is overprotective of his sister as he wants to keep her safe from danger. It was this side of him that caused Kai to decide to become a ninja so he could better protect her.
Kai was also the first ninja to learn about Nya's identity of Samurai X and expressed pride in her vigilante acts, as she happily accepted them. He also agreed to keep her secret from the other ninja who later found out.
In Season 4, he was curious over what power she might have after it is revealed by Garmadon that they'll inherit their ancestors' abilities. He also pretended to betray her, though let her know it was an act.
During Season 5, when Nya begins training to become the Water Ninja, Kai was excited that his sister was the new addition to the team. He was very impressed to see Nya unlock her true potential and defeat the Preeminent leading to the ninja achieving victory over the ghostly threat.
In "Day of the Departed," he and Nya pay tribute to their parents and wonder if they will ever learn what happened to them before the ghost of Chen attacked them. The two siblings fought against their enemy and showed great teamwork as they manage to defeat Chen once more.
During The Hands of Time, Kai informs Nya that their father was still alive and he was helping their enemies, the Time Twins, though Nya refused to believe such a thing until Kai proved it to her. When they come face to face with their father, Kai attacks him and Nya tries to stop him as he is still their father. After their mother appeared, she explained to Kai on what caused them to serve the Time Twins. Kai and Nya were soon forced to help the Time Twins when they threaten their parents and a weaken Wu. After managing to obtain the last time blade, Kai and Nya pursued the villains as they time-traveled to the past and after a long battle across Time, they manage to emerge victorious at the cost of Wu. They return to their friends and save their father, who was rapidly aging due to a blew by a Time Blade. They happily reunite with their parents after many years of being apart.
In "Green Destiny," Kai returns to Ninjago and worried for his sister's safety. Nya quickly contacts him to tell him she hears him and the siblings were reunited and they fight off the sons of Garmadon together. Afterward, the siblings celebrate their victory once Garmadon is defeated.
During "The Fall," Kai comforted Nya as she broke down at unintentionally causing Cole to fall to the darkness and encouraged her by speaking highly of her creativity and calling her the brains of the group, something that caught her by surprise. Kai's words also caused Nya to get an idea to defeat the threat, as Kai was amazed at her thinking.
In the Seabound Finale when Nya sacrifices herself to become apart of the ocean to defeat Wojira. After she remains in this state, Kai became confident they could turn her back to normal. However, Nya decided to become apart of the Endless Sea and her older brother watched with shock as she merged with the ocean. Kai then remained saddened by his sister's fate as he attended her memorial.
Ray is Kai and Nya's father. He was a blacksmith and the one who Kai inherited his powers. He disappeared sometime after Nya was born and Kai often wondered what happened to him and their mother. Kai took over his father's blacksmith shop and swore to surpass his father's skills, even though his talents were far behind his. In Season 4, he was interested in hearing the hidden story about his parents' disappearance from Chen, who promised the information if he betrayed his friends.
In "Day of the Departed," he and Nya pay tribute to their parents and wonder if they would ever find out what happened to them.
In The Hands of Time, Kai was shocked to hear from Acronix that he knew Kai's father and caused him to question everything. Later on, he is horrified to hear from Krux about his parents being traitors who aided him and his brother. Kai then begins to dive deep into his parents' past and discover that his father did in fact aid the Time Twins. Kai was angered by this revelation and when reuniting with his father, he labeled him a traitor and attacked him until Nya and his mother Maya stopped him.
Kai learned that his parents were forced to help the Time Twins after Krux threatened to harm him and Nya when they were children, which caused Kai to feel guilty over trying to hurt his parents and apologized for accusing them when they were protecting him and Nya. Kai later saved his father from succumbing to the effects of the time blades and was overjoyed to have him back after so long.
The two would meet again in Seabound when Kai requested his parents to stay at the monastery when Nya lost control of her powers. Unlike his sister, Kai enjoyed having his parents' company and he would spend time playing video games and training with his father, strengthening their bond. When Ray and Maya decided it was time to leave, Kai was sad his parents had to leave and he and his father played video games one last time before his departure.
Maya is Kai and Nya's mother. She disappeared sometime after Nya was born and Kai often wondered about what happened to her and their father. In Season 4, he was interested in hearing the hidden story about his parents' disappearance from Chen who promised the information if he betrayed his friends.
In "Day of the Departed," he and Nya pay tribute to their parents and wonder if they would ever find out what happened to them.
In "The Hands of Time," Kai was shocked to hear from Krux about his parents being traitors who aided him and his brother. Kai then begins to dive deep into his parents' past and discover that his parents did in fact aid the Time Twins. Kai was angered by this revelation and when reuniting with his father, he labeled him a traitor and attacked him until Nya and Maya arrived and stopped him.
While Kai hugged her, he still believed her to be a traitor until he learned the truth of his parents being forced to help the Time Twins after Krux threatened to harm him and his sister, which caused Kai to feel guilty over trying to hurt his parents and sincerely apologizes for accusing them when they were only protecting their family. After Kai managed to save his father from the effects of the time blades, he was overjoyed to have both his parents back after so long.
Sometime later, Kai would request his mother and father to stay at the monastery for a while when Nya's powers went out of control. Kai enjoyed his mother's company and did not mind her pampering him.
Kai first encountered her on the ferry to Chen's Island and while there he defended her from Karlof. From the moment they met, Kai immediately fell in love with her right after he got his first look at her and developed a romantic interest in Skylor but was shocked to learn she uses his element, thinking they may be related but soon learns her power is Amber, which brings back his feelings for her. During the Tournament, Kai grows to trust Skylor, especially when she shows information regarding Cole and Zane. This changes when he is shocked to learn that she is Chen's daughter and was secretly working with her father to undermine his friends, but he also believes there is a conscience in her. His feelings for her seem reciprocated, as Skylor couldn't look him in the eye when Chen captured him. She also seemed touched when he mentioned that he fell for her. Skylor soon became convinced by Kai and turned against her father and was captured as a result, but Kai went off to rescue her from Chen.
Kai's feelings were so great that he accepted her new Anacondrai form, which allowed him to unlock his Elemental Fire Dragon. When she was sad about people running from her new appearance, he cheered her up by mentioning she has changed by joining him and his friends. The two fought together to face Chen and his army, emerging victorious from the conflict. After the battle, Kai offers her a spot on the team but she declines and decides to take over her father's noodle business, promising to fully redeem herself. She tries to give Kai a farewell kiss, but Dareth interrupts them. However, she promises to keep in touch.
Near the end of Season 6, Kai meets Skylor again (she was recruited by Jay to rescue the other ninja from Nadakhan's magical sword) after he is freed from the sword. Kai thanks Skylor, saying that he is unsure how he can repay her. Skylor just replies that Kai "owes her one." However, Jay's last wish undid everything and Skylor forgot this as did Kai unless he was told by Jay and Nya.
As of "Green Destiny," they are reunited only for Kai to become worried about her upon seeing her poisoned condition. After Garmadon was defeated, he is there when she is cured and wakes up in Kai's arms, much to his joy and relief. In the end, Skylor becomes Kai's girlfriend and they share a romantic embrace.
Kai is Master Wu's student, along with the other ninja. At the beginning of the series, Kai was recruited by Master Wu to become a ninja. At first, Kai didn't care about training with Wu even considering him a crazy old man. But as the series went on, they grew closer. Kai and the other ninja cared about their master and is glad to have him mentoring them.
In Season 7, he and the other ninja come to his aid when he fights Acronix by himself when Nya saves Wu from succumbing to the effects of the time blades. When Wu is back to normal, he helps them foil the Time Twins’ plan. He later aids Kai and Nya in the battle against the Time Twins, but when the villains prepare to finish the ninja off, Wu sacrifices himself to save them. Though happy at their victory, Kai couldn't help but be worried when his master was feared to be lost in time with their enemies. He and the ninja elect Lloyd as their master and are happy to hear his first order is to discover what happened to Wu.
In Season 8, Kai and the others are still searching for Wu before dealing with the Sons of Garmadon. They find an infant and after said child starts to walk and talk, they realize he is their master de-aged because of the time blades. Kai was relieved to hear he would turn back after the blades' effects wear off.
During Season 9, while they are stranded in the Realm of Oni and Dragons Kai is the ninja who is amazed at Wu aging and wonders if his memories will retell lessons to better help him as he rapidly ages. Kai even looked forward to seeing Wu become himself again and helped him with learning how the team works in missions. After master rapidly aged into an adult and regain his memories, Kai was excited to see he was back.
In "Green Destiny," Kai and Wu fought against Garmadon's forces and were victorious in the conflict.
“I know how that feels. After I lost my dad, I lost my way. But I was lucky to have my sister watch over me. Don't worry, big shot. I'll watch over you from now on.
Initially, the ninja and Lloyd had a bitter relationship because they prevented him from ruling Ninjago. When he redeemed, Kai realized his destiny was to protect him. Though Kai was jealous that Lloyd was the Green Ninja instead of him, that doesn't hinder their friendship.
In Season 4, Kai briefly turned on Lloyd when Chen's staff corrupted him and it was revealed he was still jealous of Lloyd before he overcame the influence. When Lloyd's father was banished to the Cursed Realm to defeat Chen, Kai made a promise to look after Lloyd when Lloyd was sad about his father's sacrifice and demise.
In Season 5, after Lloyd was possessed by Morro, Kai was the most determined out of all the ninja to get their friend back. When Morro left Lloyd's body and threatened him, Kai gave up the Realm Crystal and leaped into the water (despite his fear and inability to swim) to save Lloyd from drowning. He was relieved to have his friend back and comforted Lloyd when he was upset about dropping his guard by telling him that they would defeat any threat. Kai's words proved to be true as the ninja were able to save the city of Stiix, defeat the Preeminent, and her army of ghosts. Kai grows a very huge and close relationship with Lloyd in Season 5.
In Season 7, he and the other ninja decide to make Lloyd their master after Wu is lost in time with the Time Twins. Kai most likely considers Lloyd as a close friend or even best friend following Season 5.
During Season 8: Sons of Garmadon, Kai made fun of Lloyd's crush on Harumi and offered bad advice. Other than that, they get along as Kai respected Lloyd's leadership and followed his orders as they worked together. Kai was also worried for Lloyd after Harumi's true colors were revealed and he didn't hesitate to try and give up his powers to heal Lloyd after he fought Garmadon.
For most of Season 9: Hunted, Kai was mostly in the Realm of Oni and Dragons and worried for Lloyd and the others. In "Green Destiny", Kai was reunited with his friend and they saved Ninjago from Garmadon, and Kai was happy Lloyd's power returned.
In Season 12, in "Superstar Rockin' Jay", Kai was shot off the roof from one of the drones in the game and was killed. Lloyd witnessed his death and grieved until Kai re-spawned next to him with three lives left. Lloyd was happy to see him okay while at the same time confused.
Kai and Cole are loyal buddies on great terms. They rarely bicker and get along very well because they share several common things: their confidence, courage, etc. In the earlier seasons, he and Cole were the leaders of the team and sometimes they alternate over the leadership.
In Season 5, Kai's and Cole's friendship grew stronger in this season. Kai is very sad that Cole became a ghost and attempted to move him out of his sadness. Throughout the rest of the season, he motivated Cole to never give up and to stay by his side. Overall, Kai respects Cole and they are close friends. Both would put the other ninja before themselves.
In "Day of the Departed," Kai was very happy to see Cole return to being a human.
In Season 7 Cole and Kai didn't interact much, though working as a team. Although, Cole was very worried about Kai after the latter and Nya followed the Time Twins through time, along with Wu. Cole's worries are relieved when Kai and Nya returned unharmed.
In Season 8: Sons of Garmadon, they continue to get along with Kai laughing at Cole singing to their enemies.
During Season 9: Hunted, Kai depended on Cole to rescue him and the others from the Dragon Hunters. Despite this, when threatened by enemies, Kai told them to eat Cole first but this was out of fear not malice.
In Season 12: Prime Empire, when Cole is under attack by Red Visors and a drone, and his rope is about to snap, Kai risks his last life for him by jumping off of the side of the cliff and land on the drone, shooting away from the Red Visors. In "The Speedway Five-Billion," Kai lets Cole ride on the back of his jet in the race until he steals a Whack Rat's Quad Bike. When they see Jay get surrounded by Red Visors, Cole and Kai both agree to go out with a bang and sacrifice themselves for their teammates.
In Season 13, once the team arrives in the city of Shintaro and try to sleep that night, Kai is annoyed when Cole wakes them up after wanting everyone to help find a locket of his parents that was held by a Geckle in his room and went back to bed. The next day, he looks guilty when being informed by Princess Vania that the Geckle was real and that this lead to Cole being captured inside the mountain. After a failed rescue attempt and a giant ruckus caused by the Skull Sorcerer which led to the team splitting up, Kai looks worried when Cole is being chased by the latter and fears for him when he gets blocked off from him and Zane.
Like all the other ninja, Kai initially thought Zane was weird, but when he found out that Zane was a Nindroid, he and the others supported and accepted Zane as their brother. He and Zane grow closer throughout the series. When Zane sacrificed himself to defeat the Overlord and was thought to be destroyed, Kai was greatly upset over his friend's demise that he couldn't be a ninja anymore.
In Season 4, Kai was very saddened at Zane’s death, being upset that Zane died instead of him, out of guilt. When Lloyd suggested the idea of replacing Zane, Kai, and the others protested at this with Kai even stating that without Zane maybe there was no team. When Kai and the others learned that Zane was still alive, they were very surprised at this and entered the Tournament of Elements to locate him. Kai was glad that Zane was okay and reunited with his fellow ninja. The two friends fought against Chen and his forces together and their relationship was still good.
In Season 5, Kai and Zane rarely interacted but Kai wasn't ready to follow Zane's leadership as seen when he laughed at Zane's malfunction voice most of the time. Zane respects Kai as a friend and ninja and usually follows Kai's leadership. Overall, Zane and Kai still get along great.
In Skybound, Zane placed his hand on Kai's shoulder to comfort him after Nya was struck with a dart of Tiger Widow venom and succumbed to the effects of the poison before Jay's wish undid everything.
In later seasons to still get along and mostly other the rational-minded of ninja. This was best seen in the Sons of Garmadon and Hunted.
When Zane was banished to the Never-Realm, Kai thought he was dead and was devastated. However after he learned of his survival, he and the others went there to save him.
During "Awakenings," an encounter with Boreal caused him to realize that Zane is the Ice Emperor. Although he was the first against this, he had to recall that he was corrupted by the scroll, and that caused his madness. Zane also finally remembers him and a happy moment of them together. He then snaps out of his delirium enough to help his friends.
Jay and Kai are friends on throughout the series, though they interact only occasionally throughout. Nonetheless, they do seem to care about each other.
At first, their status was generally tense and turbulent since they were fairly distant through Season 1, rarely interacting. Their status became even more strained after Kai's sister became enamored with Jay, much to Kai's disgruntled chagrin. However, he soon respected and approved of their relationship.
As of Season 2, Kai and Jay have interacted more and become more agreeable, in spite of their contrasting personalities. There are moments where Kai saves Jay from peril and comforts Jay when he is feeling down.
As the series went on, Kai and Jay have shown themselves to good friends and work well as teammates. During Season 4, Jay was aware of Kai's crush on Skylor and was amused by it and annoyed as the latter nearly let him fall.
In Season 9: Hunted, Kai was put off by Jay not freaking out when they were stranded in the Realm of Oni and Dragons and accepting of it. Kai remarked that Jay not freaking out was making him worried, though he was relieved to see him back to normal after they were captured.
Throughout Season 10, the two interact a lot more than they normally do when Jay turns to Kai when needing help on asking Nya to be his Yang. In "The Darkness Comes," when Jay is practicing his speech, Kai tries to convince him that he needs to ask in an arrogant and cocky manner. Jay declines this suggestion, which Kai agrees. In "Endings" Kai criticizes Jay's sense of timing when he finally asks the question right before they all fight the Oni one last time, but is happy when his sister says yes. He cockily comments that he taught Jay how to ask the question.
However, by the beginning of Season 11 onwards, Kai seems to have gained some sort of low opinion regarding Jay and his capabilities, an example being when he offers to let Jay be eaten by the Beohernie, saying that it should be "someone expendable" like him, he also felt worthless to the team after Jay saved him from two Elemental Cobras, something he seens ashamed of happening.
In Season 12, Kai believes his having a nightmare when encountering the League of Jay, which consists of many people with avatars of Jay himself. Later on the season, Kai willingly gave up his last life along with Cole so that his sister, Lloyd and Jay can move on to stop Unagami.
In Season 13, when Kai needs to jump from a lava cliff, he thinks he can do it if even Jay could, only to almost fall and need help.
In The Island, when Jay is about to be sacrificed to "Wojira", Kai asks who would want Jay as a sacrifice, which annoyed Nya. Still, he was one of the Ninja along with Lloyd and Nya to berate Mammatus on sacrificing Jay, implying Kai does have some respect for his team mate and friend still.
They first meet on the ferry, Kai was initially amazed to meet others like him and his friends. They became rivals after Karlof harassed Skylor and came to blows until Clouse broke it up.
During the search for the Jadeblades, they fought again and Karlof won but Kai stole the Jadeblade from him.
Because of Kai, Karlof was eliminated and was stripped of his powers, something a horrified Kai witnessed and felt guilt over. Later on, the two overcame their differences when Kai destroyed Chen's staff and Karlof regained his powers.
After defeating Chen's cultists, the two acknowledged each other as friends and promised help in the future.
During Season 9: Hunted, Karlof kept his word and helped Kai and the ninja as they battled the Colossus.
Kai holds a dislike for Chen both for kidnapping Zane and stealing his powers. He worked with his friends to undermine Chen's plan and was shocked to learn of Skylor being his daughter, as well as his spy.
Chen did not seem to mind Kai, especially after learning his daughter liked him and agreed to her request of letting Kai joined them. Chen showed the Red Ninja his collection, while Kai was irritated with him but Chen on the other hand found Kai to be amusing, since he still possessed spirit even after his powers were taken. Chen caught Kai's interest with secrets about his missing parents, something that seemingly caused Kai to help in capturing Lloyd. In truth, Kai was deceiving Chen and made this known to a captive Nya and Garmadon. Kai later revealed his ruse after turning Skylor against Chen and return the elemental powers to everyone.
In "Day of the Departed," they meet again after Chen was summoned by Yang and eagerly went after Kai and Nya as his targets, as they both were at fault for his defeat. Kai was shocked to see him again and fought against the ghost. In spite of Chen's relentless attacks, Kai and his sister defeated him and Chen was sent back to the Departed Realm.
Kai holds extreme hatred for them, since they were largely responsible for his and Nya's parents disappearance. Kai hadn't known of this until he and the other ninja fought Acronix and the latter made a vague hint at it and he became suspicious of it. Kai later confronted Krux who was revealed to have been masquerading as Dr. Saunders and told him his parents were traitors.
Kai later learned they forced his parents into servitude under threat of harming him and Nya when they were young. Kai's dislike for them boiled especially since the Time Twins forced him and Nya to serve them by threatening his parents and a rapidly aging Wu. Kai followed the Twins to the past and disrupted their plans, saving Ninjago from their influence.
In the final battle, Kai joined his sister and master in fighting them. Kai and Nya overpowered Krux before Acronix stepped in to defend his brother, nearly defeating the ninja until Wu sacrificed himself and sent the ninja back to the present while he stays behind to defeat the enemy.
Ironically Kai and Krux are somewhat similar since they both despise technology and they are the oldest child.
Upon seeing Faith as Heavy Metal, Kai disliked her as she was Iron Baron's Number 2 and best skilled fighter. He also thought of her as a big threat when knowing that she fought the First Borne and lived, getting her scale in the process and making it into a sword.
However, when Faith tracked the ninja and Wu down, with the latter finding out he was the son of the First Spinjitzu Master, she revealed she was a woman and became more honest with them, which made Kai begin to trust her, especially after realizing that she also disliked Baron. Apart from Wu, Kai and Faith seemed to have the most interactions. Faith was impressed with how Kai tripped two of Iron Barron's lackeys with a Vengstone chain, and Kai was thankful when Faith showed him how to use the Chain Gun. In the process, when Kai grew frustrated on the first few failed attempts, Faith placed her hand on his shoulder, letting him know that he must have patience.
When Faith and the First Borne arrived at the Monastery, both injured, Kai was worried for her. In "Endings" when Faith announced that she would fight with them against the Oni, Kai was skeptical as she should be resting, but went with it and they fought together.
Kai developed an instant hatred for Iron Baron, upon seeing he killed the Ultra Dragon (despite not knowing it) and turned his remains into a throne.
Their antagonism became stronger when Kai and his friends released the dragons he and his Hunters captured. Baron would set his Hunters after the ninja for their actions.
Kai and the other ninja were sought out by Baron but managed to avoid him, while also making sure that he doesn't claim Wu after he discovers the latter's heritage.
The fire serpentine was responsible for taking Kai's power and leaving the Fire Ninja powerless. For this, Kai gained a dislike for her and wants to stop her plans from coming true.
Even after defeating her, Kai still holds a strong hatred for her and laments that he is useless because of his inability to contribute in his friends' quest.
During his time in the Never-Realm, Kai moved slowly passed his sadness and was able to regain his powers, much to his joy.
Akita · Benthomaar · Cole · Cyrus Borg · Dareth · Dr. Julien · Faith · Garmadon · Jay · Kai · Karlof · Kataru · Lloyd · Misako · Nya · P.I.X.A.L. · Police Commissioner · Skales · Skylor · Vania · Wu · Zane