During his first appearances, Kai wore a dark green cloak, concealing his body and face. Underneath it, Kai is a tall, muscular man who wears his black hair in thick dreadlocks (similar to reggae singer Bob Marley). In a flashback, he was seen wearing the traditional outfit of a swordsman. He also wore earrings.
He is called by Kurei as a lone wolf, but clearly is respected by the latter, as is seen when Kurei orders Mikoto not to speak a single word against Kai after his death.
Kai is one of the most determined characters in the entire series. His attempts at bettering his skills are the defining trait of his character. Kai is also full of pride and refuses to believe a swordsman ten years younger than himself could be superior. In his flashbacks it is seen that he is hard working and firmly believes that hard work and unending dedication to swordsmanship can overpower the talent of a genius such as Tokiya. During his training in the art of Hyomon Ken, he was constantly reminded by Kyōza Meguri that Tokiya did everything Kai did, and he did it with perfection, while Kai was inferior. He was also passionate enough about winning over Tokiya that he shed a tear in joy. This constant goading pushed him to master Hyomon Ken in only one year, while it took Tokiya seven years. Despite using such a bloodthirsty madōgu, Kai himself is honorable enough to pause a battle in order to allow Tokiya to pick up his weapon, and will not strike down a defenseless opponent.
Part in story
Kai studied Hyōmon Ken under Kyōza Meguri, who also trained Tokiya. Through constantly comparing him to his senpai, Tokiya by saying that Tokiya is "gold" and he is "a rock", Kyōza instilled within Kai a sense of anger and revenge in order to show Tokiya how futile living his life only for revenge is. Kai battles against Tokiya in the final round of the tournament and manages to defeat him, fulfilling his desire to be the best Hyōmon Ken master and in order to avenge his broken pride. After telling Tokiya not to live a life of revenge the way he himself did and that it was Kyōza Meguri who killed Mifuyu, Kai stabs himself with the Hyōma En to feed its thirst and allows himself to fall into the pit below the arena.
In the anime, Kai is supposedly the first Hyōmon Ken successor, but is discarded soon after Kyōza noticed Tokiya's potential. He also says that Kyōza Meguri killed Mifuyu, but adds that Kyōza's reason for doing so is because he believed that revenge would drive Tokiya to train harder (he says nothing about Kyōza Meguri's motives in the manga). After his battle with Tokiya, he does not stab himself with the Hyōma En, but he still allows himself to fall into the pit.
Hyōma En & Hyōmon Ken
Kai wields the Madōgu Hyōma En, which can manipulate ice and is the evil counterpart of Tokiya Mikagami's Ensui. In the manga, the Hyōma En has a mind of its own and constantly demands to be fed blood, especially Tokiya's. He is able to take ice from moisture in the atmosphere using Hyōma En to continue to use his ice based attacks.
Kai is an expert swordsman that was taught the Hyōmon Ken by Kyōza Meguri.