I'll lead them and I'll shut you down if it's the last thing I do.
~ Kai to Morro
Kai is the main protagonist, later overall deuteragonist of LEGO Ninjago: Masters of Spinjitzu. He is the Elemental Master and Ninja of Fire, and Nya's older brother. Kai and Nya continued to work as blacksmiths in their missing father's shop. With the help of a wise master, Wu, Kai learnt Spinjitzu and discovered his threeotherteammates, with whom he embarked on many adventures with as they battle the Serpentine and eventually The Great Devourer.
Kai and the Ninja soon fought the Stone Army in a desperate effort to prevent the final battle, which was soon made all but certain. After Lloyd defeated the Overlord and his father was purified of evil, peace resumed in Ninjago until Pythor returned, aiding The Overlord in becoming the Golden Master and commanding the Nindroid army. The Ninja fought the Overlord and his Nindroids until Zane sacrificed himself, vanquishing the Overlord but seemingly dying in the process. The Ninja team separated until Lloyd reunited them to rescue Zane on Chen's Island. There, they competed in the Tournament of Elements before allying with the other Elemental Masters to defeat Chen's army. Soon after the war's end, Lloyd was possessed by Morro, who escaped the Cursed Realm and released a legion of ghosts. The Ninja battled Morro and his forces at Stiix before the Cursed Realm was destroyed.
The Ninja found fame after saving Stiix, with Kai becoming obsessed with his popularity. Yet they found themselves facing a new problem when Nadakhan returned with the Sky Pirates. During their first encounter, Nadakhan trapped Kai in the Sword of Souls, though he was later freed by Jay. Kai witnessed Nya die after being struck by Tiger Widow venom, though Jay's final wish reversed recent events, saving her. On the Day of the Departed, Cole accidentally revived the Ninjas' old enemies, including Chen, who attacked Kai and Nya. The siblings defeated Chen before going to Yang's Temple, where they witnessed Cole become a human.
He is voiced by Vincent Tong, who voiced Flash Sentry from My Little Pony: Equestria Girls film series, Sandbar in the My Little Pony: Friendship is Magic TV series, and Touta Matsuda from Death Note.
Fittingly for his elemental affinity, Kai is a very hotheaded boy, prone to taking action before he thinks things through. He is capable of making careful observations when the need arises, but his emotions often override his caution. His single-mindedness made interaction with his teammates difficult at first (especially due to his worry about Nya), but he eventually learned to work with the other Ninja. He can be arrogant at times, insisting that he is better than his peers and growing depressed when he seems to be lagging behind their training.
Later, following the Ninjas' fame, Kai would end up getting something of a larger head and bigger ego, constantly flirting with girls and enjoying the merchandise styled after him. Likewise, he indulged in his fame the most out of all of the ninja.
Kai also has something of a stubborn side, believing that he is in the right until overwhelming evidence to the contrary is presented. This is especially pronounced during Lord Garmadon's stay on the Destiny's Bounty; Kai repeatedly accused the four-armed villain of treachery or plotting despite Garmadon's insistence that saving Lloyd was his primary concern at the time. On the other hand, Kai's stubborn streak lends him incredible determination that allows him to bolster the team's spirit in the face of impossible odds.
The most prevalent facet of Kai's personality is his good heart and dedication to people he cares about—if someone can earn his friendship, he will stop at nothing to protect them. Much of his stubbornness and recklessness are due to his desire to help his loved ones, either directly or by becoming strong enough to defeat anyone who might threaten them with their plans. In turn, his friends and family work to balance out Kai's more extreme traits, causing him to become a levelheaded mentor for Lloyd by the time he and his comrades begin training the Green Ninja.
He is voiced by Vincent Tong, who is mostly known for voicing Touta Matsuda from Death Note. in the movie, he is voiced by Michael Peña, who is mostly known for playing Luis in the Marvel Cinematic Universe.
He did not like technology most likely due to his past upbringing but he has been seen playing video games and indulging in other technical items, meaning it has grown on him.
Kai is very similar to Tanjiro Kamado from Demon Slayer: Kimetsu No Yaiba.
They both have younger sisters respectively (Kai with Nya and Tanjiro with Nezuko).
Their fire-based abilities are inherited by their fathers respectively.
They are friends with the heroes who use lightning-based abilities and fall in love with their sisters (Jay and Zenitsu).
They are also friends with the heroes with brutal personalities and obsession with food (Cole and Inosuke).
They have parents who were taken away by their arch enemies (The difference is that Ray and Maya were captured by the Time Twins, Krux and Acronix, and Kie Kamado was killed by Muzan Kibutsuji).