Koyomi Araragi is a third-year student at Naoetsu Private High School. The eldest child of police officer parents, he has two younger sisters, Karen and Tsukihi. This upbringing fostered a strong sense of justice, though his parents' strict methods created familial tension. During childhood, he briefly hosted Sodachi Oikura, a girl from an abusive household, but failed to recognize her distress due to his naive perspective. Later, in middle school, Oikura tutored him in mathematics, sparking his lasting proficiency in the subject; again, he overlooked her suffering.

In his first year of high school, Araragi encountered Oikura during a class cheating scandal. Serving as trial chairman, he failed to control the situation, resulting in Oikura being falsely blamed via majority vote. Witnessing this perversion of justice and the teacher's betrayal deeply traumatized him. He embraced an antisocial philosophy, believing friendships "lower human integrity" by creating vulnerabilities and compromising ethics. This isolation lasted until near the end of his second year.

During spring break, Araragi encountered the near-death vampire Kiss-Shot Acerola-Orion Heart-Under-Blade. Initially reluctant, he sacrificed himself to save her after hearing her plea, becoming her thrall. Guided by occult specialist Meme Oshino, he retrieved Kiss-Shot's stolen limbs, restoring her power. In their final confrontation, Kiss-Shot revealed her intent to die to revert his vampirism. Oshino mediated a compromise: Araragi drained most of her blood, reducing her to a childlike form, Shinobu Oshino, while he retained residual vampiric traits like regeneration, enhanced strength, and sun aversion. This event intertwined their existences; Shinobu relies on his blood, and his death would restore her full power.

Post-vampirism, Araragi's personality shifted. His antisocial stance softened after befriending classmate Tsubasa Hanekawa, who persisted through his initial hostility. He developed "chronic hero syndrome," recklessly aiding others afflicted by supernatural "oddities," often exploiting his regeneration. This tendency stemmed from self-loathing and a desire for validation as a "savior." He rationalized it as selfishness, claiming he only helped vulnerable beings to feel needed.

Araragi's appearance features black hair covering one eye and grey eyes. He grows his hair longer to conceal vampire bite marks. He typically wears his school uniform or casual hoodies and jeans, noted for unremarkable fashion sense. Physically fit due to vampirism, he is shorter than his sister Karen and girlfriend Hitagi Senjougahara.

Key relationships define his development. He dates Senjougahara after resolving her "weightless" condition, forming a bond marked by abrasive banter and mutual reliance. His dynamic with Shinobu evolves from mutual dependence to deep companionship; they confide in each other during crises. Though initially idealized, his relationship with Hanekawa becomes platonic after rejecting her romantic feelings. He frequently teases his sisters, Karen and Tsukihi, though their "Fire Sisters" heroism mirrors his own tendencies. His interactions with characters like Mayoi Hachikuji and Suruga Kanbaru often involve wordplay and comedic deflection.

Araragi's residual vampiric abilities include rapid healing, intensifying after feeding Shinobu his blood. As the series progresses, his soul becomes more vampiric, causing him to lose his reflection. He later gains "aberration creation" after Shinobu absorbs a fragment of the Rainy Devil, enabling him to manifest entities like Ougi Oshino—a physical manifestation of his self-doubt and suppressed guilt over failing Oikura. Accepting Ougi represents his reconciliation with his flawed self.

In later life, Araragi becomes a police officer like his parents and marries Senjougahara. They experience multiple breakups during college but consistently reconcile. He maintains ties to the supernatural world, continuing his partnership with Shinobu.