Isshin Yuiga is a third-year student who serves as the president of the Adventure Club at the high school on the artificial island. His outward appearance might initially suggest a person with a noble or respectable cause, but this impression is deceptive, as he is primarily defined by his manipulative and treacherous nature. His driving motivation is an intense obsession with obtaining the artifacts known as Nananas Collection, as he seeks to acquire their supernatural powers for his own benefit. He is relentless in this pursuit and is known for being unpredictable, stubborn, and acting according to his own will, with a notable lack of trust in others.

In the story, Isshin plays a significant role as an antagonist and a source of conflict, often betraying his fellow club members to further his own goals. For instance, he lures Yama Juugo and the detective Ikkyuu Tensai into a ruin under the guise of a club activity. After they successfully navigate the traps to reach the treasure, Isshin uses the artifact, a magical cane, to trap them inside the ruin while he escapes with the prize. This pattern of betrayal is a recurring element of his character, as he sells out his allies on more than one occasion in his quest for power, though his schemes ultimately end in failure each time. Beyond his antagonistic role, he also acts in a more neutral capacity on certain occasions, such as serving as the banker for a forced acquisition game, where his duties require him to be impartial.

Isshin has several key relationships that define his place in the story. He shares a significant, albeit largely adversarial, connection with the protagonist Yama Juugo. In one notable incident, Isshin uses the Wizards Cane on Juugo, a decision that later forces him to confront the limitations of his own knowledge and the mistakes he has made. His relationship with the clubs vice-president, Ibara Yuu, is also notable; she is described as obsessive and possessive, and she consistently follows him, assisting in his schemes. He also has connections with figures like Sansa, for whom he is willing to act fraudulently in a game, showing that his alliances can shift based on the situation.

Despite his villainous tendencies, Isshin is not a static character and undergoes a degree of development. In the end, he comes to realize the errors of his ways, particularly after the incident with the Wizards Cane reveals a limitation he had never been aware of, forcing him to acknowledge his own failures. His abilities are largely derived from the artifacts from Nananas Collection that he manages to acquire. Among these are a teapot that can brew tea without leaves, a pair of glasses that grant the wearer the ability to copy another persons knowledge and skills, and the previously mentioned magical staff that can grant wishes.