Fiodor Dovstoïevski, more commonly known as Fyodor Dostoevsky, is a primary antagonist and a mastermind of significant influence in the world of Bungo Stray Dogs. He is the enigmatic leader of a clandestine organization known as the Rats in the House of the Dead and later becomes a founding member of the terrorist group the Decay of the Angel. Fyodor is a tall, slim young man with a pale complexion and a tired, sharp-eyed appearance. He has long, messy, purplish-black hair that reaches his shoulders and is most often seen in a formal white shirt and pants, a long black coat trimmed with white fur, and a signature white ushanka hat.

Behind his polite and composed demeanor lies a cunning, unpredictable, and profoundly sinister personality. Fyodor is a genius-level strategist whose intellect, outstanding memory, and deep understanding of human nature allow him to outmaneuver even the most intelligent of his foes, including Osamu Dazai. He possesses an almost supernatural calm, rarely faltering in dire situations and often willingly allowing himself to be captured as part of his elaborate schemes. Fyodor is a manipulator who views his subordinates and associates not as allies, but as disposable pawns to be exploited and discarded when they have served their purpose in his grand design.

Fyodor's motivations are rooted in a twisted, messianic form of religious belief. He views humanity as inherently sinful and foolish, believing that people cannot help but kill each other. He speaks of carrying out the lord’s will and seeks to purify the world of what he considers its greatest sin: the existence of gifted individuals with supernatural abilities. To achieve this "better world," he aims to acquire a legendary, reality-altering artifact known as the Book, which he intends to use to rid the world of all ability users. Despite his rhetoric of salvation and divine mission, his methods are cruel and unforgiving, leading his enemies, such as Dazai, to refer to him as a "demon" or "monster".

In the story, Fyodor operates primarily as a shadowy, behind-the-scenes antagonist. He first demonstrates his reach by hacking into the Moby Dick and forcing it to crash after the Guild’s defeat, effectively neutralizing that organization. He later instigates the Cannibalism arc by using a virus-type ability to poison the leaders of both the Armed Detective Agency and the Port Mafia, forcing the two rival organizations into a deadly standoff. His role often involves pitting his enemies against each other while he works to obtain the Book. A significant portion of his development involves his imprisonment in the high-security prison Meursault alongside his arch-nemesis, Dazai, where the two engage in a battle of wits from their cells. In recent events, Fyodor has demonstrated a shocking ability to survive death through his own power, taking over the body of the vampire leader Bram Stoker and unleashing catastrophic destruction on his enemies.

Fyodor's most notable ability is called Crime and Punishment. While its exact mechanics are shrouded in mystery, it has been shown to kill a person instantly with a single touch, and this effect is not limited to direct skin-to-skin contact. His ability appears to be intrinsically linked to his identity, as it did not manifest an independent form or turn against him when exposed to a mist that separates ability users from their powers; instead, it declared itself to be the very "punishment" for Fyodor's "crime". His key relationships are defined by rivalry and manipulation. He is the arch-nemesis of Osamu Dazai, the only person he considers an intellectual equal, and their encounters are fraught with tension and intricate psychological warfare. He also manipulates individuals like Nathaniel Hawthorne by preying on their desires, and he coldly uses Sigma, a being born from the Book, as a tool before betraying him without hesitation.