January 4 (Capricorn)
146 cm (4'9")
Themself, chaos, brown sugar
Themself, peace, society, hospitals
They wear a mini hat in a diagonal position. They wear a white shirt and suspenders with a black coat with a scarf around his neck, along with shorts with thin red stripes, and brown shoes with high white socks that reach his calves. It is shown that Yumeno carries a sling bag with an animal picture. Q always carries their doll, vital to their ability.
Q's hair is divided in half down the middle, with the left half being white, and the right being black. Their eyes are peculiar, with the right pupil being a solid yellow star, and the left being a yellow ellipse that is dark blue in the middle, the same color as their irises.
Although a child, Yumeno has a twisted, sadistic personality, and revels in the carnage Dogra Magra creates. They can play the part of an innocent, harmless child, but are quick to 180 into their darkest nature. Their sadism led to their confinement thanks to Dazai, and now Q intends on "breaking" everyone and everything to make Dazai suffer the worst pain imaginable.
Yumeno, when not under lock down, seems fairly independent, and indulges in normal activities of children their age, but in most cases is far from helpless.
After the Guild abducts them, Yumeno suffers extreme phyiscal and emotional trauma, ensnared in John Steinbeck's ability, spread throughout Yokohama, and thus forced to feel every inch of pain that the trees "feel". The pain overwhelms Yumeno, to the point they snap and curse the entire city. Their breakdown drives them to questioning God's love for everyone, and reveals their own intense hatred for humanity, themselves, and their ability, which they never asked for.
Their ability, Dogra Magra (ドグラ・マグラ, Dogura Magura?), is a mind control curse that curses his targets or 'receivers' by ravaging their consciousness with hallucinations, causing them to attack everyone in sight with no regard on whom they might be; allies or foes, friends or enemies.  The 'receivers' are those that "hurt Q", by bumping into Q and causing the razor blades taped to their arms to cut into them. This curse is triggered by mutilating a doll that they carry, which is the source. A blotch in the shape of a hand print appears on the body of the cursed individual. After the doll is mutilated, it apparently regenerates.
Thus far, the only way to stop Q's curse is with No Longer Human.
Q joined the Port Mafia shortly after Osamu Dazai, who's tasked with finding out the child's ability. It's implied Ōgai Mori kept him under his watch. Due to an unknown incident that led to a massive amount of mafia casualities, Dazai had Q locked away.
PlotŌgai Mori releases Q during the three-way conflict, using them as part of the bait scheme to drag both the Guild and Armed Detective Agency down. They board the same train as Kirako Haruno and Naomi Tanizaki. The girls run into them in one of the train cars and apologise, to which Q insists they're the one who should say sorry. The girls later introduce Q to Atsushi Nakajima once he arrives to retrieve them. Q intentionally bumps into Atsushi, and shows off the gruesome razors piercing their skin.
Q's doll laughs maniacally, floating over their head, and tears its own head in half. Q entertains themselves watching Dogra Magra work on Atsushi, making him hallucinate Naomi and Kirako as the ones being affected and attacking him.
By the time Dazai stops Atsushi and nullifies Dogra Magra, Q reboards the train just as it starts up, and comments how Dazai's new friends are easy to break. Q says this is fine, as they'll save Dazai's breaking for last, intent on breaking everything and forcing Dazai to go through the worst pain imaginable. According to Dazai, confining Q turned out to be a harrowing task, leading to a massive death count for the Port Mafia. At the end of his patience, Dazai declares he'll tear Q's heart out the next time they meet. Q smiles and laughs, shutting the train door and escaping.
A week after Francis Scott Key Fitzgerald abducts Atsushi, Q continues to enjoy life outside confinement. They wonder who else they can "play" with, the meaning ambiguous, but recalls that Mori told them not to "play" with anyone until finishing their work even though they're bored. 
Q locates Howard Philips Lovecraft, asking him to play. Attempting to humour them, Lovecraft tries mimicking balloon animals, but naturally his tentacles merely make a grotesque display that grosses Q out. Q decides to harm themselves on Lovecraft, telling him to show them how insane they get. His body contorting, Lovecraft reveals his true form to Q, terrifying them and leading to their abduction. Locked away in an abandoned building by Steinbeck, Q suffers relentless torture and pain thanks to Steinbeck's Grapes of Wrath, which writhes through Q's body. The agony drives Q's state of mind to dangerously unstable levels.  Grapes of Wrath connects Q's pain and the 'pain' trees feel when harmed by citizens, essentially making their pains one and the same. As such, anyone harming trees and other flora receives Q's curse. At their breaking point, Q bursts into tears, lamenting their misery.
They never asked for their ability, and don't understand why bad things always happen to them. Steinbeck indifferently explains that Q suffers simply because they were born, and even though God exists, God doesn't love Q. The exchange ultimately breaks Q, who activates Dogra Magra's curse.
After Atsushi recovers the doll from Fitzgerald and has Dazai nullify the ability, Q apparently loses consciousness, with Lovecraft and Steinbeck guarding them, aware the Port Mafia and Armed Detective Agency will come to rescue them. In the aftermath, the Port Mafia suffered 18 deaths, and over 100 other causalities related to sub-organisations.
Assigned to save them, Dazai and Chūya Nakahara revive their Twin Dark status. Once he reaches Q, Dazai threatens to kill them, but Chūya tells him to go through with it. holding a grudge against the deaths Q's ability causes. Dazai bluffs, as for as long as Q stays with the Port Mafia, no one can afford to kill Dazai - making Q insurance for Dazai's life. Chūya recovers Q, but Lovecraft attacks.
Not until Dazai and Chūya defeat Lovecraft is Q returned to the Port Mafia. They have not been seen since.
- Q's demented nature, the hallucinations their ability causes, and their dislike of hospitals all stem from the novel Dogra Magra, featuring an amnesiac psychiatric patient that endures surreal, horrific experiences and experiments in an asylum.
- In the same way that Edgar Allan Poe inspired Ranpo Edogawa in real life, HP Lovecraft inspired Yumeno Kyusaku in real life.
- ↑ [Interview w/ Asagiri Kafka & Sango Harukawa]
- ↑ Bungo Stray Dogs WAN! CH. 54
- ↑ 3.0 3.1 3.2 Bingo Stray Dogs manga, volume 7, chapter 25, page 35
- ↑ Bungo Stray Dogs Chapter 25 page 17, 23
- ↑ Bungo Stray Dogs light novel, Fifteen, epilogue
- ↑ Bungo Stray Dogs manga, volume 6, chapter 24, page 150
- ↑ Bungo Stray Dogs manga, volume 7, chapter 25, page 12
- ↑ Bungo Stray Dogs manga, volume 7, chapter 27, page 103
- ↑ Bungo Stray Dogs manga, volume 7, chapter 27, page 107
- ↑ Bungo Stray Dogs manga, volume 7, chapter 28, pages 118
- ↑ Bungo Stray Dogs manga, volume 7, chapter 28
- ↑ Bungo Stray Dogs manga, volume 8, chapter 30
- ↑ Bungo Stray Dogs manga, volume 8, chapter 31