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 white 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, blonde 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.
Joi tries to twist the labyrinth of tunnels as much as he can while Izuku continues to smash through the wall. Joi sees Himiko and Twice, so he separates the heroes and police into two separate groups so Himiko and Twice can take them on and then protects them by dropping a wall in front of them when they are forced to retreatHe becomes irritated as he observes Himiko and Twice interacting.
Himiko and Twice then start throwing insults at Joi, causing him to lose his temper and he desires to crush them all to death. As a result, he starts twisting the labyrinth to the point where Izuku can hear him more clearly and pinpoint his location. He then smashed through where Joi is hiding, exposing him. Shota immediately uses his erasure quirk to disable Joi’s mimicry, leaving him to Ella helplessly to the floor as Himiko and Twice tauntingly wave goodbye to him. As Joi rages at Himiko and Twice, Sir Nighteye throws one of his High Density Weights at Joi, knocking him unconscious, and Izuku catches Joi. After Joi is tied up and comes to his senses, he continued to scream about Himiko and Twice for betraying them. He was taken into police custody after the battle. 
Overall Abilities: Being a member of the Shie Hassaikai, Joi is a very formidable individual. He also possesses a great handle on his Quirk, Mimicry, that he uses to merge with objects of specific sizes, take control of them, and clandestinely attack his opponents. His overall Quirk handling enables Joi to excel in espionage, making him the stealthiest member of the Yakuza, as well as making him effective in launching surprise attacks on enemies. When he's under the effects of the Quirk-enhancing drug Trigger, Joi's Mimicry is augmented to where he was capable of manipulating the underground portion of the Shie Hassaikai Compound in order to prevent Sir Nighteye's raid team from progressing further. Joi accomplished this task by twisting corridors, creating walls, and tossing people into isolated rooms, while also launching attacks of his own, and because he was hidden inside the concrete, Eraser Head's Erasure Quirk could not be used to stop the integrated yakuza. Eventually however, Himiko Toga provoked Joi with taunts about the Shie Hassaikai, making him lose focus, enabling Izuku Midoriya to locate him, giving Eraser Head the opportunity to finally erase Joi's Quirk, coupled with Sir Nighteye completely incapacitating the exposed villain with a Hyper-Density Seal.
Mimicry (擬態 Gitai?): Joi's Quirk allows him to merge his body and mind into an object, granting 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.