Kai is the current Elemental Master and Ninja of Fire, Nya's older brother, Skylor's boyfriend, Ray and Maya's son, and mentor of Wyldfyre. He and his younger sister worked as blacksmiths in their father's blacksmith shop Four Weapons all their life until they met Master Wu. With his guidance, Kai learned Spinjitzu and discovered his other teammates, Cole, Jay, Zane, and Lloyd, with whom he embarked on many adventures, battling the likes of the Serpentine and the Stone Army. After Zane's sacrifice to defeat the Overlord, the ninja parted ways before reuniting to go rescue Zane on Chen's Island. There, Kai developed a crush on 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 Anacondrai Cultists. After the war ended, Lloyd was possessed by Morro, who escaped the Cursed Realm and released a legion of ghosts. Kai and the ninja battled Morro and his forces before the Cursed Realm was destroyed. The ninja found fame after saving Stiix, but when Nadakhan returned with the Sky Pirates, Kai was the first of the ninja trapped in the Sword of Souls. Jay eventually freed him and the rest of the ninja, and reversed these events with his final wish. On the Day of the Departed, Kai and Nya fought a revived Chen, who they defeated once more. When the Time Twins returned to Ninjago, Krux tried to convince Kai that Kai's parents were traitors. Kai and Nya reunited with their parents and stopped Krux and Acronix from changing the future, but at the cost of losing Wu in the Time Vortex. A year later, Kai and the ninja watch over Princess Harumi after she loses her adoptive parents. They later discover she is secretly the leader of the Sons of Garmadon and wants to resurrect Lord Garmadon. Despite the ninja's efforts, Harumi succeeds, and shortly after Kai is marooned in the First Realm with Cole, Jay, Zane and a de-aged Wu. Kai and the ninja help save the dragons from the Dragon Hunters, and help Wu regain his confidence and memories. After Iron Baron's defeat, Wu reclaims the Dragon Armor, in turn allowing them to return to Ninjago. Kai and his allies take down Garmadon's Colossus, restoring peace to Ninjago once more. Not long after, the malicious Oni invade Ninjago, forcing the ninja to ally with Lord Garmadon. Kai uses his blacksmithing skills to reforge the Golden Armor into the Golden Weapons, and later defeats the Oni by using the Tornado of Creation with the other ninja, Wu, and Garmadon. Months later, Kai and his friends fight Aspheera and her army, after accidentally releasing them from their tomb, and Aspheera seizes Kai's power with her staff. After Zane is banished by Aspheera's sorcery, Kai and the ninja search for him in the Never-Realm. While Lloyd goes to find Zane, Kai and the others protect the Ice Fishers from the Ice Emperor's Blizzard Samurai, and he regains his element in time to combat Boreal. The ninja learn a memoryless Zane was manipulated into becoming the Ice Emperor. He returns to his senses and frees the realm from its eternal winter, and the ninja return to Ninjago. Over the next few months, the ninja face numerous new threats. They enter the virtual world of Prime Empire when hundreds of players are zapped into the game. During their quest to stop Unagami, Kai and Cole sacrifice themselves in a race, but everyone is restored to reality when Jay helps Unagami and his creator make amends. Kai and the ninja later traveled to the Kingdom of Shintaro, where Vangelis discreetly terrorizes the Geckles and Munce as the "Skull Sorcerer". The ninja struggle to bring peace to Shintaro, but are eventually captured. After being freed, they fight the Re-Awakened and defeat the Skull Sorcerer. Kai and the ninja later travel to the Island of the Keepers to save Misako and Wu. There, the ninja discover the Storm Amulet and the existence of Wojira - an ancient serpent that Kalmaar of Merlopia seeks to unleash. The ninja journey to the Endless Sea to investigate, but are unable to stop the beast's awakening, and Ninjago City is flooded. Ultimately, Nya sacrifices herself and becomes one with the sea in a bid to defeat Wojira. A year after Nya's loss, Kai has opened up his own dojo. Zane and Cole find him in the dojo teaching kids combat skills. While discussing a plan Kai accidentally knocks over a glass of water. This angers Jay and he attacks Kai. Kai fights back with twice the anger. When Nya manifests in a water form, they devise a plan to drain her power and restore her body. The ninja break Aspheera out of Kryptarium and steal her staff; she restores Nya, but the ninja are arrested for their actions. They escape a prison bus and set off to learn of the Crystal King, who assembled a council of the Ninja's worst villains - including a revived Harumi. In the ensuing chaos, the Crystal King - revealed as the Overlord - unleashes an army of Crystal Warriors to ravage and corrupt the people of Ninjago. The final battle commences, with the ninja and all their allies through the years coming together to fight The Overlord, his confidants and his army. The ninja combine the Golden Weapons, releasing their powers in the form of the Golden Ultra Dragon, which helps Lloyd vanquish the Overlord. The ninja and their allies celebrate their greatest victory, and come together to rebuild the monastery. Several weeks later, Kai went on a journey with Skylor to defeat a tribe worshipping Fire Fang and regains his powers in the process. When The Merge occurred, where all realms formed into one, Kai woke up on an unfamiliar island, plagued with many terrifying monsters and spent weeks trying to find his way out. As he roamed that strange land, he struggled to keep himself from giving in to despair. During his travels in the Land of Monsters, he encounters a dragon called Daidan, who he stopped from ennacting vengence on a monster by killing its unhatched egg after it killed her own child, and Rusty, a sentient Fire mech that used to belong to a previous Fire Master who turned into a monster. After Rusty sacrificed himself to save Kai and Daidan from a Sea Monster, Kai was able to regain his hope and continue his journey back to Ninjago. Weeks later, Kai managed to find his way to Ninjago and the monastery, reuniting with Lloyd. Taking the Bounty, Kai went on a journey to study the combined realms while keeping Lloyd up to date, until he stopped upon reaching the Realm of Madness, reuniting with Nya and helping to defend the Craglings from invading dragons. Eventually, Lloyd and his new students, Arin and Sora, and a dragon called Riyu find them and help them, allowing Kai and Nya to return with them. While Lloyd and his students went to Imperium to stop their use of Photacs, Kai remained at the monastery, where he was called towards the mysterious Monastery of Imperium, where he and Nya meet up, find Zane, and help Lloyd, his students, Riyu, and Wyldfyre escape. While searching for the Dragon Core in Wyldfyre's old home, Kai came across the Lava-tides, who were miffed by Wyldfyre's pranks, until they manage to get the core. However, Dorama managed to swap it with a fake beforehand. And while waiting for Heatwave to heal, Kai taught Wyldfyre patience, eventually taking her under his wing. After discovering the fake, Kai and Wyldfyre, along with Lloyd, Zane, and Arin return to Imperium, where they're recruited by a resistance to expose Empress Beatrix's lies before she used a weapon to create a MergeQuake storm, which Kai was sent into a rift but was brought back after Lloyd stabilized the merged realms and during one lesson, Kai discovered his and Wyldfyre's powers are different. Sometime later Kai helped other ninja to handle Lobbo being mutated, stopping the Mechanic, and stopping the Lava-Tides from stealing dragon eggs. A year later, news of Ras in the Cloud Kingdom reached Kai and the other ninja. They went on a mission to stop him, but were defeated by Cinder. To counter Shatterspin, they learned about the Dragon Masters and headed to their dojo to request training to learn Rising Dragon, with Kai being the first to achieve it. They trained until the Blood Moon arrived. To infiltrate the Ritual of the Blood Moon, Kai, Sora, and Wyldfyre disguised themselves as Wolf Mask Warriors. However, Ras trapped Kai in the Nether-Space. Despite the ninja's efforts, Kai remained trapped with Bonzle while Nokt was freed. For some time, Kai was trapped with Bonzle in the Nether-Space. They had to hide from the other Forbidden Five and saw them talk to an outside ally. Kai saw that he still had Riyu's scales on his outfit, which allowed him to enter into a connection with him. Thanks to the Dragon Ivory stone, and to Riyu who managed to establish a link between Nya and Kai thanks to his dragon power, Nya was able to communicate with the latter. Nya was able to use her connection with Dragon Ivory to blast the dragon horns, opening a breach in the Nether-Space, allowing Kai and Bonzle to escape. Nokt was about to attack Nya when Kai intervened, and fought against the enemy. Nokt, possessing all the powers, took the upper hand and was going to finish him off when Roby and Wyldfyre used the Elemental Cup to deprive him of his powers and return them to their original owners. The Forbidden Five, Arin and Ras end up marooned in a different realm after the portal gate they entered was destroyed. After all this was over, Nya explained to Kai about Jay, and Kai promised to help her find him. After the events on the Tournament of Sources, the Ninja split up to look after their missing members, with Kai, Nya and Wyldfyre going to look for Jay. Despite Kai managing to find a clue and give hope to Nya, the trio is forced to postpone their search after Kur attacks the City of Temples and steals Roby's life force, leaving him seriously ill. While there, Kai and Nya learn about the Forbidden Five's objective of freeing Thunderfang by collecting all five Prismatic Blades. After reuniting with the others, Kai helps in the fight against Thunderfang. After the latter is defeated, he and the others are reunited with Jay, who is revealed as being the bounty hunter going after Ras and later reveals that Arin's parents are alive. While Lloyd and his group search for Arin's parents with Jay's help, Kai, along with Cole, stays behind to help training Frak in Spinjitzu. During it, they discover the missing Prismatic Blades and then follow the same lead Master Wu gave Cole previously, which results in the trio unearthing the remains of the deceased Source Dragon of Balance. When returning to the Land of Lost Things, they find it being attacked by Nokt and Spectral Dragonian Warriors, with them taking the souls of Bonzle, Fritz and Spitz, proceeding to attack Imperium's capital city. Kai leads the trio to protect the city and its inhabitants, with Percival telling them he could upgrade the Prismatic Blades in their possession to be effective on ghosts. After an argument with Cole about what should take priority, Kai proceeds to follow the plan to upgrade the blades, along with Frak. After they are ready, they fend off the enemy, but not in time of saving Cole. Returning to the monastery, Kai decides to help the others in awakening the Arc Dragon of Focus to defeat Thunderfang once and for all.
