Kyusaku Yumeno, often referred to by the nickname Q, is a young member of the Port Mafia. They appear to be around thirteen years old and possess a small, childlike frame. Their most distinctive physical features include hair that is split down the middle, with the left side white and the right side black, and mismatched eyes where the right pupil is a yellow star shape and the left is a yellow ring. Despite their youthful appearance, Q is regarded as one of the most dangerous and unstable individuals within the organization.

Q's personality is characterized by a deep-seated sadism and a love of chaos. They take genuine delight in the suffering and destruction caused by their ability. At the same time, they are capable of shifting their demeanor in an instant, acting like an innocent, harmless child when it suits their purposes before revealing their darker, more malevolent nature. This unpredictability, along with a hedonistic streak and a general disdain for authority, makes them difficult to control. Their motivations are largely driven by a desire to alleviate boredom and to inflict pain on others, particularly those they blame for their confinement, such as Osamu Dazai. Despite their cruel exterior, there are hints of a deeper turmoil. After being abducted and subjected to immense suffering by the Guild, Q experienced a breakdown that revealed a profound self-hatred and resentment for their own ability, which they never wanted.

The central element of the character is their ability, named Dogra Magra. This ability is a powerful mind-control curse that affects anyone who physically harms Q. Q intentionally tapes razor blades to the inside of their sleeves. When a person accidentally or deliberately bumps into them and gets cut, a hand-shaped mark appears on that person's body. This marks them as a receiver of the curse. Q then mutilates a doll they carry, and upon doing so, the cursed individual is overwhelmed by intense hallucinations that force them to experience their worst nightmares as reality. This drives the victim into a berserk state where they attack anyone in sight, friend or foe, indiscriminately. The doll used to trigger the curse is self-regenerating, allowing Q to use it repeatedly. The ability is considered so dangerous that Osamu Dazai, whose own ability can nullify others, personally orchestrated Q's sealing within the Port Mafia to prevent further catastrophe.

Q's role in the story is that of a volatile weapon and a wildcard. They were originally sealed away by the Port Mafia after causing significant casualties within the organization, as even their allies deemed Q too demented to be left free. They were later released to be used as a tool in the conflict against the Armed Detective Agency and the Guild. During this period, Q's sadistic nature was unleashed, creating widespread panic and chaos. However, Q was later captured by the Guild, where they were subjected to severe physical and emotional torture by John Steinbeck's ability, forcing them to feel immense pain. This traumatic experience pushed them to a breaking point, leading them to curse the entire city of Yokohama in a fit of despair and rage. The key relationship in their development is with Osamu Dazai, who is both their captor and the only person capable of reliably neutralizing their ability. Q harbors a deep resentment for Dazai and seeks to make him suffer the most pain imaginable. Throughout their ordeal, Q shows little to no attachment to their allies in the Port Mafia, viewing them merely as tools or obstacles. While Q does not undergo a moral redemption, they do experience a significant shift from a purely sadistic agent of chaos to a more tragic figure whose torment exposes the deep wounds beneath their monstrous actions.