Mahiru Seki is a childhood friend of both Kou Yamori and Akira Asai, and he is portrayed as one of the most popular and capable students in his class, excelling in both academics and sports. He lives with his family, who run a local florist shop, and he is often seen helping them there while wearing his school uniform, which consists of a gray blazer over a red hoodie, gray pants, and white tennis shoes.

On the surface, Mahiru is cheerful, friendly, and responsible, with an outgoing nature that makes him loved by everyone around him. However, this bright personality is largely a facade, a mask he developed following the tragic death of his older brother, Shougo. After his brother died in an accident, Mahiru’s home life fell apart; his mother was consumed by grief and became emotionally neglectful, while his father focused all his attention on supporting her, leaving Mahiru feeling isolated and lonely. To cope, the once timid and quiet child began copying his late brother’s personality, striving to live up to the image of the perfect student his brother had represented. Despite being surrounded by adoring classmates, Mahiru does not genuinely enjoy their company, finding true connection only with his old friends Kou and Akira.

His life changes dramatically after meeting a vampire named Kiku Hoshimi. Mahiru develops deep romantic feelings for her, and she becomes his emotional support and confidante, listening to his worries and making him feel understood for the first time since his brother’s death. Even after learning that she is a vampire, his affection never wavers, and he eventually wishes to become her subordinate, a familiar. He even jokes with Kou about who will achieve this goal first. However, this relationship leads to a drastic and disturbing change in his personality. As he falls further under Kiku’s influence, he becomes rude, blunt, violent, and even enthusiastic about harming humans, a shift that is later revealed to stem from his distress over Kiku’s impending death.

The pressure of his situation causes Mahiru to disappear from his friends’ lives for a while. His relationship with Kou becomes particularly strained, evolving from a friendly rivalry into open conflict. He feels betrayed when Kou suggests he is being used, and he admits to being incredibly jealous of Kou for being special. Despite this turmoil, their bond is resilient. Mahiru secretly values a photograph of himself with Kou and Akira too much to burn it, and eventually, the two friends reconcile, with Mahiru apologizing for his actions.

Throughout the story, Mahiru struggles with a profound sense of loneliness and a desire to escape his painful everyday life. His motivation is not simply to gain power, but to find a place where he truly belongs and to escape the emotional neglect he suffers at home. This neglect is most evident in his relationship with his mother, who was so broken by the loss of her firstborn that she could not properly care for Mahiru, even refusing to let him go on a school trip because his brother was not going. Ultimately, Mahiru’s story is a tragic exploration of coping with loss and seeking genuine connection, as his love for Kiku and his desire to become a vampire propel him into supernatural events beyond his control, leading to his final decision to die with her rather than live without her.