Koyomi is generally a laid-back individual. Before the events of his last spring break, where he encountered Tsubasa Hanekawa, he was notably asocial during high school, believing that forming friendships would “lower his integrity as a human.” This belief stemmed from a negative experience during his first year. Over time, different accounts of this philosophy emerged. In Koyomi Vamp, he explains that closeness to others creates vulnerabilities if friends are hurt, he too would suffer. In Ougi Formula, a slightly different view is presented: Koyomi expresses discomfort with the idea that “truth” can be dictated by the majority, regardless of its accuracy. In both cases, his reasoning supports the notion that isolation would make him stronger.

His encounter with Hanekawa prompted him to gradually open up and form connections with others. Academically, Koyomi struggles, nearly failing all his classes except mathematics. At home, he perceives his family relationships as strained, although his younger sisters, Karen and Tsukihi Araragi, view their family more positively.

Koyomi also displays a strong familiarity with anime, a trait noted by Nadeko Sengoku. Several characters, particularly Mayoi Hachikuji and Hitagi Senjougahara, occasionally tease him by suggesting he might have lolicon tendencies. Despite his flaws, Koyomi has a sincere desire to help those around him and feels a deep sense of responsibility for issues he could potentially influence. This inclination often leads him into reckless behavior, particularly when he pushes the limits of his regenerative abilities. Others around him sometimes view his willingness to help as overly soft-hearted or selfishly motivated.

Koyomi is depicted as a young man with black hair, gray eyes, and a medium complexion. His hair typically covers his left eye, although in the Kizumonogatari films, it is shown covering his right. Following the events of Koyomi Vamp, he decides to grow out his hair to better conceal the bite marks on his neck. Throughout the chronological progression of the anime, his hair length subtly changes short in Bakemonogatari, slightly longer in Tsukimonogatari, and reaching greater lengths by Shinobumonogatari.

Outside of his school uniform, Koyomi dresses casually, usually wearing a hoodie paired with jeans. His sisters often tease him for his poor fashion sense. A distinctive feature shared between Koyomi and his sisters is an expressive ahoge (a strand of hair that sticks up), which shifts in shape to reflect his emotions, an element that was not part of his original design by VOFAN.

Physically, Koyomi is fit due to his partial vampiric nature, although his build is generally only noticeable when he is not wearing a shirt. In the light novels, he is described as being somewhat short. Hitagi Senjougahara is noted to be slightly taller than him, a detail Koyomi humorously worried would be evident in the anime adaptation, although they are usually portrayed as similar in height. He is still a bit taller than Tsubasa Hanekawa but noticeably shorter than his younger sister, Karen Araragi, who is exceptionally tall for her age.

Koyomi is the eldest of three siblings, with his younger sisters being Karen and Tsukihi Araragi. Their parents, both police officers, are often away from home for long hours, which likely influenced the strong sense of justice shared among the siblings. During most of his high school life, Koyomi lived as an outcast and was a poor student, struggling academically except in mathematics, a subject he excelled in thanks to early tutoring from Sodachi Oikura. Following a difficult experience during his first year, he developed the belief that forming friendships would weaken him, deciding that staying isolated would preserve his strength as an individual.

Before the events of the series, Koyomi was attacked by a vampire during spring break and was transformed into a vampire himself. Although Meme Oshino later helped him recover much of his humanity, Koyomi retained several vampiric traits, including enhanced vision, superhuman strength, and regenerative abilities. He remains wary of sunlight, although it no longer physically harms him. During this incident, he also encountered Tsubasa Hanekawa, whose kindness and influence encouraged him to reconsider his stance on friendship.

In his third year of high school, Koyomi is appointed vice president of his class at Tsubasa’s suggestion, as part of an effort to help him turn his life around. This role puts him in charge of organizing events and handling class affairs alongside her.

Karen Araragi and Tsukihi Araragi
Koyomi shares a close, though sometimes chaotic, relationship with his younger sisters. Despite frequent teasing and physical altercations, he cares deeply for both Karen and Tsukihi, often involving himself in their personal problems. Their bond reflects a mix of sibling rivalry and strong familial affection.

Tsubasa Hanekawa
Tsubasa played a pivotal role in encouraging Koyomi to open up to others and reconsider his isolated lifestyle. As classmates and later as class officers, they develop a trusting relationship based on mutual respect and shared experiences, although underlying tensions and complexities occasionally surface.

Hitagi Senjougahara
After saving her from a supernatural affliction, Koyomi forms a deep romantic relationship with Hitagi. Their bond is built on honesty, vulnerability, and sharp-witted interactions. Hitagi becomes one of the few people Koyomi trusts completely, and she plays a major role in his personal growth throughout the series.

Shinobu Oshino
Initially the vampire responsible for Koyomi’s transformation, Shinobu and Koyomi eventually develop a unique, symbiotic partnership. Although their relationship is often marked by playful antagonism, it is underscored by loyalty and a deep, complicated connection forged by their shared history.

Mayoi Hachikuji
Koyomi and Mayoi develop a friendly, light-hearted relationship characterized by humorous banter. While their interactions often appear comedic, Koyomi displays a protective side toward Mayoi, treating her like a younger sibling.