Kai

Kai is the red ninja of Fire. His sister is Nya. He was chosen to protect the Green Ninja (Lloyd Garmadon) along with the other three Ninja. He is the only character known to have a sister. Kai was the last ninja to unlock his true potential.

Early Life
Kai, along with his sister, were the children of the blacksmith and owner of Four Weapons. When their father died, Kai took it upon himself to keep the family business going and swore he would become an even greater blacksmith.

Way of the Ninja
When the Four Weapons Blacksmith shop was attacked by Samukai and his Skeleton army, Kai attempted to defend it with some samurai armor he made. The armor was slow and clunky however, and he was soon subdued, and would have died were it not for the quick action of Sensei Wu. Wu then told Kai about The First Master of Spinjitzu and his two sons who fought over the four Golden Weapons, explaining that a Honest Man had been entrusted with the map to their resting places and the honest man was he's father. Sensei Wu took Kai to the Mountains of Impossible Height in order to train him to be a Spinjitzu master so he could save his sister, and all of Ninjago. During his training, Kai had to learn patience and how to harness the "fire" inside of him. This training consisted of running a course before Sensei Wu finished his tea. Kai accomplished this after several days by knocking the teacup out of Sensei's hands when he was about halfway through the course with his wooden training sword. He then told Kai to get some rest, and that his final test would come in the morning.

The Golden Weapon
Later that night, while Kai was pondering Sensei Wu's training and brushing his teeth, he was set upon by three black-suited ninjas. Kai was able to fend them off for a while, disorienting them on the training course. Before they could fight much longer, however, Sensei Wu arrived and told them all to stop, at which the three ninjas revealed themselves as more of Wu's students. Wu used Creation to give each of the ninjas a suit that matched their elemental properties, and introduced them to each other. He then told them what they needed to do to defeat Lord Garmadon, who is Wu's brother. Cole interjected and asked if they were still to learn Spinjitzu first. Sensei then informed them that they each already knew it, but it must be unlocked at the right time. Then Wu said that they would take the horse carriage(pulled by Kai and his fellow Ninja) to the Caves of Despair, the location of the first Golden Weapon, The Scythe of Quakes. At the Caves of Despair, Kai ran ahead of the other ninjas while Cole was formulating a plan. The other three ninjas chased after him, and caught up to him just before the chamber where the Scythe was housed. Jay then accidentally revealed their position to the skeleton army digging for the weapon, and they were soon in a battle. After Jay discovered the key to unlocking his Spinjitzu, Kai and the others were quick to follow. They used it to drive off the skeleton army, after which they discovered that they had bigger problems, namely, the weapon's guardian, the Earth Dragon. Kai happened to be holding the Scythe at the time, and decided that the only way out was to use it, contrary to Sensei's instructions. After escaping, Kai was strictly repremanded by Sensei Wu, and they went onward toward the next weapon, The Shurikens of Ice.

King of Shadows
While on their way to the Frozen Wasteland, the location of the Shurikens, Wu told the ninjas about the most powerful technique in Spinjitzu, the Tornado of Creation. Kai began to attempt it, but was stopped by Wu, who explained that it could have disastrous concequences if done incorrectly. When the ninjas arrived at the location, they discovered several skeleton warriors frozen in ice, and deduced that they had been the second people to get there. When they discovered the Shurikens suspended from the celing, they lifted Zane up to grab them. When he did, a block of ice formed around him, and the Ice Dragon awoke. The ninjas then used Zane as a sled to escape, and went to the hiding place of the Nunchucks of Lightning, the Floating Ruins. On top of the chain, Jay found the Nunchucks, as well as the Lightning Dragon. As they were being chased by skeletons, the ninjas used Jay's wing-glider invention to leap off the chain and get away to the Forest of Tranquility. While they were sleeping that night, Kai thought he saw his sister Nya. He got up and chased her to the Fire Temple, where he discovered that it wasn't really Nya, but a shadow deception created by Lord Garmadon. His sister was in the Fire Temple however, only suspended over a pit of lava. Kai had to remove the Sword of Fire from its pedestal to cut the chain and save his sister, which was Garmadon's plan all along. He cintrolled Kai's shadow, and used it to defeat him. The shadow couldn't be hurt by Kai, but Kai could be hurt by the shadow. Garmadon made multiple copies of the shadow, which made quick work of Kai, and obtained the Sword. All would have been lost if Sensei Wu hadn't intervened at that moment, using his own shadow to defeat Garmadon's Shadow Warriors, regaining the Sword. Garmadon then decided to awaken the Fire Dragon, knowing the guardian would impede the band. The dragon blocked the entrance to the temple, which meant it would be easy for Garmadon to bring the other three weapons to the temple and unite them there, as Samukai had ambushed the other ninjas. Wu then sacrificed himself to go to The Underworld, in order to keep the weapons apart.

Weapons of Destiny
Kai and Nya discovered that the Dragon only wanted to protect the Sword, and the ninjas used the guardians to go to the Underworld to help Sensei Wu. The Dragons preformed their variation of Spinjitzu and crossed over, landing not far awat from the Skulkin Fortress. They tried to make their way overhead by grabbing onto the roots of plants only to discover that they were spider legs. Out of fear they dropped down to the floor below, where they were noticed by Nuckal. They were going to defeat the Skulkin until the Spiders landed on the stone surface also. Thinking fast Jay remembered the Tornado of Creation. And although Zane protested about disatrous concequences, they used the Tornado, successfully defeating they're enemies. They arrived in the fortress to witness Sensei Wu being defeated by Samukai. Samukai acquired all of the Golden Weapons once but in doing so destroyed his body and transformed him into a portal which Lord Garmadon quickly escaped through. Sensei Wu assured him about Lord Garmadon and the ninjas departed the Underworld. Kai and his new friends went back to the Blacksmith shop and were greeted by Nya and several villagers.

Flight of the Dragon Ninja
Cole, Zane, Jay, and Kai were told to by Sensei to train but, instead decided to hold a Dragon race. Zane felt that they should be training but, Kai told him riding your dragon "was kinda training." Kai and the other ninjas raced for a while but Cole won when he "activated the turbo" by throwing food in front of his Dragon's nose. Although he won Kai said that they'd get him the next day.

The New Masters of Spinjitzu
While Kai and the others were sleeping Nuckal and Kruncha stole his sword in hopes of preforming Spinjitzu. They turned on each other though and while they were fighting Kai came out half-asleep and walked by the still pair and turned of the training course that they activated. Believing that the training course turned itself on, Kai yelled to Jay to fix it, but Jay said that he'd "do it tommorow." When Kai was back asleep the two Skulkin put the weapons back in their places since they couldn't control them.

Return to the Fire Temple
The Fire Dragon, Flame flew Kai and Nya to the Fire Temple, wanting to show them something. After dismounting Flame led Kai to a passage connecting Ninjago to the Underworld. Kai started planning what to do when the ground fell out from beneath him. Krazi noticed him and alerted Wyplash, who sent several Skulkin after him. He dissarmed his enemies, but they picked him up. Nya and Flame then shot a blast of heat scaring the Skulkin and sealing the passage.

Rise of the Snakes
When reports of Lord Garmadon started surfacing, the Ninja were tasked with going to Jamanaki village to find the source of the problem. They found out it was Lloyd who caused the trouble and tied him to the top of a sign, embarrasing him. When they went back to the Dojo to train Sensei Wu told them to return to Jamanaki village because the Hypnobrai had been released. Kai and Cole let Nya fight with them as an "Honorary Ninja." Nya got the Hypnobrai staff to the fountain and healed the bewildered and hypnotized villagers, before the ninjas drove the snakes into retreat.

Home
The Ninjas were training when they pointed out to Sensei Wu how Zane seemed strange. When the Mailman arrived Kai and the others discovered Zane was orphaned. The next day Kai and the others went to Lloyd's tree-house which acted as a Hypnobrai base. They started destroying it, but a hypnotized Cole turned on them. Before long Sensei Wu came riding in on Flame playing an ancient flute, which cancelled the Serpentine's powers. The group returned to the monastary on Flame right as the Serpentine set it on fire. Zane and Shard put the fire out but, the monastary was beyond repair. Kai and the others verbally attacked Zane, but Sensei Wu caught them reminding them that Zane was they're team mate and had them apologize. But before they could Zane flew off on Shard to follow the Falcon. Kai and the others ate around a fire until Nya noticed Zane entering the camp, at which point he led them to there new home, Destiny's Bounty.

Appearance
Kai wears a red ninja outfit, and has brown, spiky hair. He usually has a very determined look on his face. His Minifigure also has some scars on its face.

Personality
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Kai is very tempestuous, and often leaps into things before he thinks. He would leap into a lake before checking to see if there's fire in it. At the beginning when he first starting training, he was less interested in teamwork and patience, because of his insistance on finding his sister, but eventually learned to get along with the other ninjas. He is a kind person who cares greatly for his friends and is very protective of his sister Nya. Other characters have noted he has an inflated ego. Although he may be an "act first-ask questions later" type, he'll come through in a pinch. And in the Finale, it seems he has matured greatly and encourages the team to keep fighting.

Weapons
Kai currently uses the Sword of Fire, but once wielded a normal katana. In his spinner sets, Kai would also use a double bladed dagger, a flail, a Double scythe, a spear, and a Golden katana.

Trivia

 * Sensei Wu found Kai last.
 * Kai thought that he was the Green Ninja (Lloyd).
 * Kai was the last ninja to unlock his true potential.
 * His name Kai means in Hawaiian "Water", as opposed to his element: Fire.
 * Zane and Kai are the two ninjas whick they have gold zx armor.
 * His hairstyle is currently the only one, which isn't available as an actual LEGO piece.