Fyodor Dostoevsky is a tall, slim young man with a pale complexion and long, messy dark hair that reaches his shoulders. His dark, sharp eyes often give him a tired appearance, and his formal attire typically consists of a long-sleeved buttoned white shirt, white pants, a long dark coat with white fur trim, a white ushanka hat, and maroon long boots. He is known to possess a weak anemic constitution and has a habit of biting the tip of his thumb until it bleeds. He also plays the cello.

Very little is known about Fyodor's past before the events of the story. At some point in time, he was captured and executed by the vampire Bram Stoker, an event which first revealed the true nature of his ability, as he used it to take over the body of his executioner. He eventually became the leader of a criminal organization called the Rats in the House of the Dead and later a founder of a more powerful group known as the Decay of the Angel. He allied himself with the Guild and the Order of the Clock Tower in a mutual effort to obtain a legendary and mysterious artifact known as the Book, though these alliances were merely a means to his own ends.

Fyodor is an enigmatic and profoundly sinister individual whose cunning and intellect are comparable to Osamu Dazai's. He is a genius mastermind, displaying incredible patience, observation, and a deep understanding of human nature, which he uses to manipulate even the smartest of his enemies. He is calm, confident, and composed in even the most dire situations, possessing enough courage to allow himself to be captured as part of his elaborate schemes. Despite his cruel and unforgiving actions, which have earned him the epithet "Demon" from his adversaries, he speaks in a polite, discreet, and occasionally amiable manner. He possesses a strong religious belief and views himself as an agent of God's will, speaking often of the sinful nature of humanity and expressing a desire to purify and free people from their sins, which he believes can only be achieved through death. This twisted sense of salvation drives him to commit acts of terrorism with complete nonchalance, as he believes he is carrying out a divine mission to create a better, more harmonious world.

Fyodor's primary motivation is to obtain the Book, a blank book in which anything written becomes the truth. His ultimate goal is to use this artifact to rid the world of all ability users, whom he considers a sin that must be cleansed. To achieve this, he first aims to exterminate the two primary ability-user organizations in Yokohama: the Port Mafia and the Armed Detective Agency. Believing that people are sinfully stupid and will continue to kill each other even when they know they are being manipulated, he sees himself as the divine instrument who must purify them.

In the story, Fyodor operates as a major antagonist. He first appears as a shadowy figure allied with the Guild, later taking control of the Moby Dick. He masterminds the Cannibalism arc, where he infects the heads of the Port Mafia and the Armed Detective Agency with a virus-type ability that forces the two organizations into a deadly, mutual destruction standoff. He later becomes a key member of the Decay of the Angel, a group that frames the Armed Detective Agency for terrorism. He is eventually captured and imprisoned in the European facility Meursault, where he continues to engage in a psychological battle of wits with his captors.

Fyodor's key relationship is his intense rivalry with Osamu Dazai of the Armed Detective Agency. The two share a past and view each other as intellectual equals, with Dazai often being the only one who can anticipate Fyodor's moves. Fyodor sees their similarities, while Dazai calls him a demon. He views his subordinates and allies, such as Nathaniel Hawthorne, Ivan Goncharov, and Sigma, as disposable pawns, manipulating their desires for his own benefit and betraying them without hesitation. For example, he brainwashes Hawthorne and later attempts to have Sigma killed after he serves his purpose.

Fyodor's ability is named Crime and Punishment. For a long time, it was believed to be a power that could kill a person with a single touch, regardless of barriers like gloves. However, it was later revealed that this was a deliberate misdirection. The true nature of his ability is far more insidious: it activates not when he touches someone, but when he himself is killed. Upon his death, his ability allows him to transfer his consciousness into the body of the person who killed him, essentially taking over their body as his new vessel. This ability is unique in that it did not turn against him when exposed to Tatsuhiko Shibusawa's mist, which separated other ability users from their powers. Instead, the manifested ability declared itself to be the "punishment" to Fyodor's "crime," stating they were closely linked. This power makes him an extraordinarily difficult antagonist to permanently defeat.