Gaku, a major antagonist, originated from the Al-Kamar Orphanage, a secret Japanese Association of Assassins (JAA) facility that trained orphans as potential Order members, though the project ultimately failed. He endured regular abuse under caretaker Sei Asaki, who used him to coerce fellow orphan Kei Uzuki (Slur) into committing killings. Uzuki eventually stormed the orphanage, eliminated the guards, and freed Gaku and the other children. Following this rescue, Gaku joined Uzuki's organization, known as X's Organization or Slur's Group, becoming its youngest member dedicated to overthrowing the JAA. Physically, Gaku is tall and muscular with spiky, slicked-back bluish-gray hair and black eyes (depicted as hot pink in the manga), accented by red eyeliner at the outer corners. His body is covered in scars, and his hands are typically bandaged. He wears black square-shaped earrings, a high-collared navy blue jacket with a yellow button, navy blue baggy pants, and white sneakers with light grey stripes. He consistently carries a large, cube-headed mace wrapped in bandages. After a confrontation with Takamura during the JAA raid, he lost his right ear, two fingers on his left hand, and his entire right hand. While Kashima later reattached his fingers and hand, his ear remained missing, replaced by a circular patch. Gaku's personality is defined by extreme calmness and a laidback demeanor; he rarely shows agitation, even in life-threatening situations. He frequently analogizes events to video games, describing fatal injuries as a "speedrun to death" or combat as akin to playing titles like Resident Evil. This nonchalance extends to snacking or gaming mid-battle. Despite this casual exterior, he exhibits brutal efficiency and cold pragmatism, dispatching JAA members without hesitation or remorse. He disregards collateral damage, as seen when he overexerted Shinaya's body while remotely controlling him, and derives enjoyment from combat, believing killing should be savored. Key relationships exist within his group. Uzuki, who saved him from the orphanage, serves as his pivotal leader and mentor. Kumanomi, another orphanage survivor, treats him with protective, almost sisterly devotion; she reacts emotionally to his injuries, aggressively pursuing those who harm him, like attacking Order member Hyo after Nagumo severed Gaku's ear. In combat, Gaku demonstrates exceptional prowess, described by Nagumo as the strongest opponent he ever faced. His abilities include enhanced strength (disemboweling opponents with single strikes), speed (outrunning machine-gun fire), endurance (fighting through dismemberment and stab wounds), and durability (withstanding attacks lethal to ordinary humans). He wields his mace with masterful proficiency and employs precise hand-to-hand techniques, such as a devastating one-inch punch. His weapon, a spiked cubic mace capable of firing lasers, was destroyed by Takamura but later replaced. He also carries a PlayStation Portable, used during downtime or mid-conflict. Key battles define his trajectory: during the JAA raid, he slaughtered dozens before Takamura severed his ear, fingers, and hand; he later fought Nagumo at a museum, sustaining lethal injuries but battling until defeat; he participated in group assaults against Takamura, ending in loss; his final known engagement occurred during the Assassin Exhibition of the Century arc. His name "Gaku" (楽) means "ease" or "comfort," reflecting his relaxed outlook. He ranked 9th in the manga's first popularity poll and is a self-professed fan of the Resident Evil series.