Kai

The Red Ninja and Master of Fire. Kai was once a blacksmith and owned a small shop with his sister Nya originally run by their late father. When the Skulkin Army took Nya hostage, Kai was forced to master Spinjitzu and eventually teamed up with the other ninja to seek the four fabled weapons of Spinjitzu to stop the evil Lord Garmadon. He was led into a trap on their quest at the Fire Temple, where Nya was held captive and threatened to be killed if Kai didn't achieve the Sword of Fire. Having no choice, Kai obtained the sword and freed his sister, only to have Garmadon's shadow attempt to steal it. Luckily, Sensei arrived and defeated his brother's evil scheme, and travelled to the Underworld to keep the weapons from uniting. After Garmadon's defeat, the Sword of Fire was thereafter used by Kai, along with Flame the Fire Dragon. Impatient and fierce in attack, Kai is potentially the most dangerous of the ninja, and almost never obeys Cole's plans.