Raised in Yokohama's slums as an orphan with his sister Gin and other children, Ryūnosuke Akutagawa suffered lung damage from the harsh environment, resulting in chronic coughing and a perpetually pale, sickly appearance. After criminals murdered their companions, his quest for revenge ended when he encountered Osamu Dazai, who had already executed the perpetrators. Dazai presented a choice: join the Port Mafia or gain financial security. At age 14, driven by rage and burgeoning respect for Dazai, Akutagawa chose the mafia, becoming Dazai’s subordinate.

Physically, he possesses a slim frame with short, uneven black hair featuring white-tipped bangs reaching his chin. Sharp gray eyes, often underlined by dark circles, dominate his features. His signature attire includes a long black coat over a white ruffled shirt, black pants, dress shoes, and a white jabot. This coat channels his ability, Rashōmon. Transformed into a vampire by Bram Stoker, his eyes developed black sclerae with vein-like patterns, and he adopted a modified black coat with a turtleneck collar.

Akutagawa adheres to a "survival of the fittest" creed, declaring the weak deserve death to cede space to the strong. He embraces his role as the Port Mafia’s "dog," committing violence unflinchingly for its objectives. Typically composed, he exhibits explosive anger when provoked—particularly regarding Dazai or Atsushi Nakajima. His resentment toward Atsushi stems from Dazai’s perceived favoritism, as Akutagawa relentlessly seeks his former mentor’s validation. Despite his brutality, he avoids gratuitous torture and displays flickers of integrity, sparing Atsushi during emotional vulnerability or apologizing to subordinates like Higuchi for disruptions. His relationship with Gin remains consistently warm, assisting her without hesitation.

His ability, Rashōmon, manipulates clothing—primarily his coat—into a shadow-like entity that slices matter and space. Techniques include:
- **Agito (Jaws)**: Manifesting colossal jaws to bite or pierce.
- **Murakumo (Swarm)**: Extending clawed appendages for long-range assaults.
- **Space Rupture**: Generating barriers by consuming space, requiring time to activate.
- **Sawarabi (Budding Bracken)**: Erupting ground spikes.
- **Higanzakura (Early Blooming Cherry Blossom)**: Enveloping targets in tendrils before impalement.
- **Demonic Armor**: Wrapping Rashōmon around himself for defense.
The ability’s versatility excels in mid-range combat, but Akutagawa avoids clothing-free situations like bathing due to this dependency.

His rivalry with Atsushi shifts from hostility to reluctant mutual respect. He refrains from killing Atsushi as the latter mourns the orphanage director, recognizing shared anguish. Their cooperation against foes like Fitzgerald and Goncharov cements this dynamic, though Akutagawa maintains his intent to eventually defeat Atsushi. His self-worth hinges on Dazai’s approval, culminating in his sacrifice against Fukuchi. Mortally wounded, he uses Rashōmon to carve an escape for Atsushi, declaring actions—not words—as his legacy.

In the BEAST timeline, Akutagawa joins the Armed Detective Agency after failing to protect Gin from the man in black (Dazai). He dons a frayed gray coat taken from a slain outlaw, symbolizing his unending vengeance quest. His core traits persist: contempt for firearms—rooted in his friends’ deaths—and a pragmatic view that only retaliation deters evil. He interacts with Kenji Miyazawa during farm work and confronts Atsushi and Kyōka Izumi, condemning their shared history as killers.

Personal details include his use of the archaic pronoun "Yatsugare," reflecting historical speech and classical literary ties. He dislikes dogs, baths, broad beans, and mandarins, while favoring antique calligraphy, paintings, tea, and figs. His chronic lung condition, born from slum pollution, reinforces his awareness of mortality.