We're the Yakuza, so you'd better not look down on us!! We're gonna rise up again to take society's control from the shadows! The Yakuza's due for a revival! That's our bedridden boss's greatest dream, and we're gonna make it happen!! You're not gonna wanna miss it! So be grateful, you losers!!Joi Irinaka to Himiko Toga and Twice in "Don't get mad, Irinaka"
Joi is usually seen inside a black suit that he possesses with his Quirk as a means to disguise himself. It appears to just be a small black cloak with short leg and arm-like protrusions, and a paler plague mask, that he wears as part of the Shie Hassaikai, sewn over where his face should be.
Outside of the bodysuit, Joi is a tall and muscular man. He has visibly pointed facial features, including a spiked nose and long, square-shaped chin, as well as a notably large mouth. He has quite long, light-colored hair that’s worn spiked up and a matching goatee.
He wears typical Yakuza clothing in the form of a dress shirt and pinstriped pants, also sporting suspenders.
As the general manager of Shie Hassaikai, Joi is responsible for controlling and administering the business activities of his organization. He is often seen counting stacks of money bills, implying that he also plays the role of the organization's treasurer.
Joi, like all members of the Precepts, is ultimately loyal to Overhaul, and can become enraged if people show disrespectful or threatening behavior towards his boss. Joi is hot-tempered and easily lets his emotions run wild, even if it costs him his stealth advantage.
When Overhaul starts fighting the League of Villains, members of Shie Hassaikai come to his aid, with Joi riding in on another member's shoulders. Tomura Shigaraki decides not to continue the fight, for which Joi mockingly compliments him.
Later, sitting next to Kai and counting money, Joi attends a meeting between the Yakuza and Tomura at the Shie Hassaikai's base. Tomura complains about the tunnel system, but Joi explains that it is how the Yakuza have survived. He then asks if Tomura will really ally himself with them. When Tomura reaches into his jacket, Joi extends his arm from his cloak and grabs hold of Tomura while Chronostasis holds a gun to Tomura's head. Wanting to hear what Tomura has to say, Overhaul orders Joi and Chrono to back off.
When the Pro Heroes and the Police Force invade the Shie Hassaikai Compound, Joi takes a drug to enhance his Mimicry and takes control of the underground tunnels. Although Mirio Togata escapes by phasing through the walls, Joi drops the Heroes through the floor, separating them from the bulk of the Police Force. When Tamaki Amajiki defeats Toya Setsuno, Yo Hojo, and Soramitsu Tabe, Joi tries to separate Shota Aizawa from the others, but Fat Gum and Eijiro Kirishima push Shota out of the way and are shoved through a hole in the wall.
Mimicry (擬態 Gitai?): Joi's Quirk allows him to merge his body and mind into an object, allowing him full control over the object as if it were his own body. Joi can only use this power on objects the size of a refrigerator or smaller.
Black Cloak: His usual attire. Joi possesses it through his Quirk in order to move around and interact with others.