Sigma originates from the supernatural world of Bungo Stray Dogs. He possesses long, straight waist-length hair, parted down the middle: lilac on the left side and white on the right. His attire includes a long white coat with a sparkly or galactic design on the interior, black pants, a shirt, tie, and a turtleneck underneath, along with earrings on both ears. His eyes are greyish, sometimes noted as yellow. Born from words written into the supernatural "Book," he awoke with no memories in a vast desert, possessing only the clothes on his back and a train ticket from a non-existent station. Confused and lacking any sense of identity, purpose, family, or friends, he wandered until captured by an armed group. During captivity, he discovered his ability and was exploited before escaping, leaving him feeling nihilistic and adrift. Fyodor Dostoevsky later approached him, inviting him to join the organization Decay of the Angel. Accepting, Sigma was entrusted with managing the Sky Casino, a floating establishment outside any nation's jurisdiction. This role provided his first sense of purpose and belonging; he developed deep loyalty to the casino and its patrons, viewing it as his home and reason for living. He demonstrated strong leadership skills, effectively managing staff and operations, and possessed an exceptional ability to memorize details of up to 20,000 guests. Despite projecting public confidence and composure, especially during crises, he privately struggled with profound insecurity and anxiety over his perceived lack of natural talent, expressing envy towards those with strong convictions. His ability enables him to immediately trade information upon physical contact: he receives knowledge he desires in return for providing information the other person seeks. He used this ability to obtain information about a "page" from the Book after an encounter with Special Division for Unusual Powers director Santōka Taneda, claiming it was merely a threat without harmful intent. Confrontations involving the Armed Detective Agency, the Hunting Dogs, and his own group unfolded. When Hunting Dog Teruko Ōkura demanded the casino's closure, he fiercely defended it, emphasizing its importance to his existence and questioning if a "common man" could protect what he cherished. He faced Teruko despite being physically outmatched, driven by desperation to preserve his anchor. Later, during a prison escape attempt orchestrated by Nikolai Gogol involving Dazai and Fyodor, Sigma became a pawn. Dazai selected him as part of a challenge, leading to a situation where a gravely injured Sigma confronted Fyodor directly. In a final act of defiance and seeking answers about his existence, he touched Fyodor, forcing an information exchange. This act caused him to lose consciousness immediately afterward. His current status and whereabouts remain unresolved. Throughout his journey, his core motivation remained the search for belonging and purpose. He stated: "Apart from the casino, I have nothing... And for the sake of that casino... I'm willing to do anything," and "In the end, I never did understand what I was born for." Creator Kafka Asagiri noted Sigma's relative "averageness" and straightforward motivations compared to other characters.