Kai is a ninja with elemental powers, he is was the Master of Fire also he had a sister named Nya Master of Water, he was main character in Ninjago LEGO for the whole time but further along he was the 2nd youngest ninja in the team as he was in 2011 was his age 14. He worked at his parents Blacksmith shop with his talent was Blacksmith, when he made his armour he had his sister with him, he was so responsible for her sis.
Kai met Sensi Wu in the battle when his sister was kidnapped by the skeleton army to get forced by Lord Garmodon as Wu brother so, Sensi Wu had tried to make Kai train well because he was the chosen one to be Red ninja, but also he failed 5 in one day but next day Kai finally flighted with the unknown ninjas.
Until he finds himself as a complete ninja he was amazed about his mate ability it was Spinjiztu the creator was the first Spinjiztu of Ninjago of history also he was passed away.
Kai was the oldest brother, and his sister Nya was the youngest. Kai was the son of Maya and Ray also Nya, Kai was with Nya when they went to the park with there parents but even there parents was token away to be forced to build armours for axronixcs army of snakes for the revenge of Sensi Wu.
Well Kai was responsible for his sister until she needs to know things when she was too young so Kai had to teach Nya so they both will survive there life and wish everything is better on back.
Kai has the power of Fire also it matches his suit of red ninja, he choosen his father elemental powers of fire, also he can be powerful if he uses his anger to get his power level updated.
He did learn one thing from his side of mates but now there friends they learnt spinjiztu there power has level them to level 1.