Chūya Nakahara's origins lie in a government experiment that fused him with the singularity Arahabaki around age seven. This fusion unleashed destruction, annihilating the facility and birthing Suribachi City. The event erased his memories prior to that age and fragmented his identity. Rescued by an unknown party, he joined the self-defense group Sheep at eight, becoming their protector through his gravity-manipulating ability, Upon the Tainted Sorrow. He felt a strong responsibility for Sheep, though he held no formal leadership role.

At fifteen, after a Port Mafia attack on Sheep, Chūya pursued rumors of Arahabaki's return. This investigation brought him into conflict with Port Mafia members Osamu Dazai and Ryūrō Hirotsu. Port Mafia boss Ōgai Mori forced Chūya into a partnership with Dazai, using the Sheep as leverage. Cooperating to uncover his origins, Chūya encountered Arthur Rimbaud. Rimbaud revealed Chūya's role as Arahabaki's vessel, and his dying words urging Chūya to embrace his humanity deeply impacted Chūya's self-perception.

Following Rimbaud's death, Sheep betrayed Chūya, aligning with the GSS and stabbing him. Surviving but cast out, Chūya accepted Dazai's subsequent offer to join the Port Mafia. His decision was partly influenced by Mori's possession of documents detailing his artificial origins as a potential government-made clone or modified human. These documents were withheld until Chūya attained executive rank, ensuring his loyalty. He received Rimbaud's hat as an initiation gift, symbolizing his new allegiance.

Rising rapidly to become a Port Mafia executive, Chūya is renowned as the organization's strongest martial artist. His ability allows him to manipulate gravity upon touch, enabling flight, projectile redirection, and environmental control. He possesses a more destructive state, Corruption, unleashed by chanting verses from Nakahara's poem "Sheep Song." This form unleashes uncontrollable power, ravaging his body and requiring Dazai's nullification ability to end safely. Chūya reserves Corruption for dire emergencies, removing his gloves only when necessary.

Chūya's personality blends surface abrasiveness with underlying loyalty. He is quick-tempered, particularly when provoked about his height or heritage, and revels in combat against worthy opponents. Yet he demonstrates reasonableness in negotiations and avoids unnecessary brutality. His hatred for Dazai—rooted in Dazai's manipulation of Sheep and constant mockery—is a recurring vulnerability, though he acknowledges Dazai's tactical prowess during their partnership as the "Double Black" duo. Chūya's loyalty runs deep to whichever group he belongs to, be it Sheep or the Port Mafia, and he treats subordinates with fairness.

Throughout his development, Chūya grappled with doubts about his humanity stemming from his inability to dream, his connection to Arahabaki, and the revelation of his artificial origins. These conflicts surfaced during encounters with figures like Rimbaud and his brother Paul Verlaine, who sought to reclaim him as a "perfect being." Despite these struggles, Chūya ultimately chose to define himself by his actions and relationships over his origins. His resolve solidified after the events of Storm Bringer, where he prioritized protecting Yokohama over existential questions.

In the comedic spin-off "Bungo Stray Dogs Wan!," Chūya appears in episodic, lighthearted scenarios alongside other series characters. This adaptation does not contribute to his canonical background, abilities, or character development, focusing instead on situational humor without advancing his narrative arc.