Kai

Kai is the Former Elemental Master and Ninja of Fire, and Nya's older brother. He and his younger sister worked as blacksmiths in their father's blacksmith shop all their life until they met Master Wu. With his help, Kai learned Spinjitzu and discovered his other teammates, with whom he embarked on many adventures; battling the likes of the Serpentine and the Stone Army. After Zane's sacrifice to destroy the Golden Master, the Ninja parted ways before reuniting to rescue Zane on Chen's Island. There, Kai grew closer to Skylor, one of the tournament contenders, as they competed in the Tournament of Elements. Ultimately, he allied with the other Elemental Masters to defeat Chen's army. After the war ended, the ghost Morro was awakened and unleashed a legion of ghosts but was eventually stopped by the Overlord. They then battled Naddakhan the Djinn with the support of Jay but was easily trapped inside the Djinn Blade. After Kai was freed, he battled several previous villains led by Sensei Yang. Yang sent Kai to Cloud Kingdom to face Kozu and Morro but was able to escape thanks to Cole. Kai then fought the Time Twins and found his long-lost parents in battle. After defeating Anubis's gang, the ninja battle the Sons of Garmadon. Led by Mr. E, they succeed in bringing back Mr. E's father using the Oni Masks, and Kai is killed by S.O.G. member Ultra Violet. Kai's place as the Ninja of Fire would be succeeded by Oni Kai.

Movie
The ninja are leaving Chen's noodle house when a stranger named Ronin appears and Kai's powers are stolen. Kai goes with Zane, John, and Jay to try to find Ronin. Back at the temple, the ninja get a message from Ronin saying they should not interfere, which the ninja ignore and the ninja split up to find Ronin. Kai, Zane, John, and Jay go to the Ninjago Ocean to look. They go on Ronin's ship and are attacked by Kronos, the Time Ninja. Kai gets a call from Nya and the Ninja head to the temple. The ninja are then attacked by Morro, who unleashes a small army of ghosts and uses his powers to take Christopher, Noah, E, Wu, Nya, L, and John. Morro then leaves, saying that he will destroy Ninjago. The remaining ninja search for the Weapon of the Time Master to find Ronin and Kronos, which Jay quickly finds. Back in time, the ninja have a run-in with Past Zane and Kai in Way of The Ninja. It is revealed to Past Kai that Nya will be reunited with Kai and it is revealed to Past Zane that he will become titanium. They then travel to a serpentine gathering from The Snake King, where the serpentine are stopped by the present ninja. They then arrive at Ninjago City guarded with Nindroids. Past Kai is on a mission and the present ninja arrive and stop the nindroids. They then arrive at a time somewhere in Tournament of Elements, most likely The Day of the Dragon, where Past Jay and Lloyd are surrounded by Cultists. The two defeat them but the cultists then turn into Anocondrai, and it is revealed to the Anocondrai by the present ninja that they will be defeated in the Corridor of Elders. They then travel to the Monastery of Spinjitzu in Battle Between Brothers, where Past Wu and Garmadon fight. They leave there too. Then they travel to a time between Legacy Of The Green Ninja and Rebooted. Ronin and Kronos are there, and Rune uses a spell to destroy Ronin's obsidian glaive, where it is then revealed that Ronin's plan was to stop Morro. Ronin, Kronos, and the ninja team up and follow Morro, only to find their temple has been destroyed. Kronos tells the ninja they are experiencing a side effect of time travel: When they travel to the past, the present moves forward in time, and that it had been four days since they were last in the present. Behind them is a portal to the Cursed Realm, and they need to destroy it. They travel to a ghost camp, where a dragon breathes on Lloyd, letting Lloyd be possessed by a ghost. The ninja find Wu, Nya, and their students. Rune and Noah are possessed, and the rest escape the Cursed Realm, however, Kronos's location is unknown and Morro and his army of ghosts arrive. Possessed Lloyd and Noah ride on Morro's dragon and arrive as well. The dragon creates more ghosts, and they surround the ninja.

Ghost Chase
Kai and Jay, on hover boards, are chased by the ghost Wrayth, when Kai trips on a rock. An injured Kai quickly hides before Wrayth came.