He has one sister, Nya(Samurai X). Before Kai became a Ninja, he was a blacksmith. Skeletons attacked him and grabbed his sister. He was trained separately from the other ninjas, Cole, Zane, and Jay, and his sole porpose was to get Nya back. he was the Main character of season 1.He was the last to reach his true potential.
In the episode called "The Green Ninja" he reached his true potential by realizing he's not the green ninja,but the protector of the green ninja(Lloyd Garmadon).His elemental weapon is the sword of fire.
Once Kai was a blacksmith,along with Nya(his sister). one day, an old man came and was searching for stuff for ninja, but all he found was Samurai armor. later that day the Skeletons attack,under Command of Lord Garmadon. Garmadon sended Samukai to retrieve a Map once made by Wu. Samukai meets wu and a battle starts. Kruncha & Nuckal got the map and they kidnapped Nya. kai failed and was angry at Wu. wu told him about the 4 Golden weapons of spinjitzu:The scythe of quakes,the Nunchunks of Lightning,the shurikens Of ice,and the sword of fire. weapons so powerfull, No one could handle all of their power. he also explains that garmadon wants to posses the 4.
Wu trains Kai a lot at the Monastery,however, all what Kai does is Fail. that Night, he finds 3 ninjas and fights them. until they are interupted by Sensei Wu. wu explains that they are there students to. wu gives them new suits,colorfull and powerfull. Jay is blue the master of lightning, Kai is the red ninja of fire,Zane is white,master of ice with a sixth sense, and Cole is Black,Master of Earth. together the ninjas find the Caves of Despair,where the Scythe of Quakes is hidden. Jay Unlocks Spinjitzu. Cole Takes the Scythe with him. however, the Earth Dragon(later named Rocky) attacks. the ninja are able to escape and meet up with Sensei Wu.
Next,the Ninjas go to the Frozen Wastelands using a Junk ship,where the Shurikens of Ice are. Zane takes the shurikens,but is frozen the ice Dragon Awakes and is chasing the ninjas. the Ninja run away and go to the Floating ruins where the Nunchunks of lightning is. the lightning dragon(Wisp) attacks and the ninja see the skeletons,too. Jay takes the Nunchunks and the ninjas escape using Ninja Gliders. they then find the Forests of tranquillity the ninja take some sleep there. Kai awakes and finds Nya. he follows her into the fire temple. but, he finds out it was a trick and he sees Nya Captive. he saves Nya and finds the Sword of Fire and sensei helps the Two. Meanwhile, at the camp the Skeletons attack and take the weapons. sensei sacrifies himself and takes the sword to the underworld. garmadon informs Samukai that, and the skeletons go to the underworld. Jay,Cole,Zane chases them but fails they go to the fire temple and meet up wit the rest. the fire dragon(Flame) Awakes and the Ninjas Use the Dragons to go to the Underworld. but finnally Nuckal Yells"NINJAAAAAA!!!!" and the Skeletons attack. the Ninjas form "The Tornado of Creation" and defeats them. the ninjas go into Garmadon's Fortress and sees Samukai and Sensei fight. Sensei loses and Samukai has all 4 Weapons. the weapons where too powerfull,that, Samukai exploden and turned into a Portal. Garmadon walks into the Portal. The Ninjas choose their one Golden Weapon. the Ninjas Return To Kai's Village and are welcomed as Heroes.