Kai Kasai is an eighteen-year-old male and a high school senior, which places him among the older participants in the deadly game at the center of Tasuketsu -Fate of the Majority-. His birthday falls on August 20th, and his blood type is O. He possesses special privileges known as mortgage and surface rights, which grant him considerable authority and influence within the rules of the competition.

In the story, Kai Kasai’s motivations become clear during the second part of the narrative, when he witnesses the overwhelming power of the first Emperor, referred to as the Koutei. Recognizing the might of this figure, Kai resolves to abandon any adversarial position and instead become a loyal aide. To achieve this goal, he leverages his sharp negotiation skills to successfully secure the role of the Koutei’s right-hand man. His primary role in the plot is therefore that of a strategic subordinate, consolidating his standing in the third part of the story to become an indispensable lieutenant.

Despite his dedication to the Koutei, Kai Kasai’s personality shows unexpected complexity. While logic would make him a strict enemy of the protagonist Linne, he occasionally breaks this dynamic by offering her advice or performing light-hearted actions, such as asking for her signature. This suggests that beneath his loyalty to a powerful master, he retains a degree of personal judgment and perhaps even a reluctant respect for his opponents. His notable ability is his talent for negotiation, which he uses as a primary tool to advance his position and navigate the treacherous politics of the game. Through these actions, Kaidevelops from a witness of power into a key player who serves a dictator while maintaining a unique, almost contradictory rapport with the opposing side.