Mahiru Seki is the childhood friend of Kou Yamori and Akira Asai. He projects an image of a popular, academically and athletically gifted middle school student with a cheerful and reliable personality, a facade he developed after the death of his older brother, Shougo Seki. His usual appearance includes a school uniform of a gray blazer, a red hoodie, gray pants, and white tennis shoes, complemented by his darker faint skin tone, black eyes, and dark blonde hair often tied in a small ponytail. After meeting the vampire Kiku Hoshimi, his appearance shifts; he lets his hair down and adopts baggy clothes, looking perpetually tired. Beneath his popular exterior, Mahiru harbors depression and trauma from severe emotional neglect at home following his brother's passing. His mother, Mrs. Seki, became apathetic and unable to accept Shougo's death, treating Mahiru as if he were invisible. His room remained unchanged from its elementary school state, and he was forbidden from activities like school trips under the pretext that his deceased brother could not participate. This treatment fostered deep resentment toward his parents, though he initially excused their behavior by believing his mother was broken from grief. He once genuinely loved working at the family flower shop but later grew to wish it would vanish along with his household. Mahiru's life transforms after meeting Kiku Hoshimi at the flower shop. He develops a deep romantic connection with her, feeling understood and fulfilled for the first time. Despite witnessing a vampire attack on his friend Akira, his loyalty to Kiku leads him to desire becoming her vampire servant to escape his painful human life. This ambition creates a friendly rivalry with Kou, as both aim to become vampires. Under Kiku's influence, Mahiru's personality shifts radically; he becomes rude, blunt, violent, and enthusiastic about killing humans, a change partly fueled by Kiku's vampiric charm, which induces euphoria and impairs his clear thinking. Yet he secretly retains a photograph of himself with Kou and Akira, defying Kiku's orders to burn all his past belongings, indicating a lingering attachment to his former life. His relationship with his mother culminates when Kiku forces a confrontation. After Kiku reveals that Mrs. Seki was willfully ignorant of Shougo's death, Mahiru confesses that he once hated her intensely but now feels nothing. He taunts her with a smile before leaving his family permanently. In a later letter to Kou, he admits he initially planned to kill his parents after turning into a vampire but changed his mind, hoping they would move on and live happily. Mahiru's story concludes when Kiku reveals she is dying and wishes to become human again by having Mahiru, whom she loves, kill her. Despite suspicions she might be lying, Mahiru accepts her plan due to his profound love for her. He chooses to die with Kiku rather than live without her. Before turning to dust alongside her, he apologizes to Kou and Akira. It is revealed that Kiku first met Mahiru years earlier, shortly after his brother's death, and comforted him when he confessed his hatred for his neglectful parents. His relationships define his character. With Kou, he shares a history as childhood friends but experiences envy and conflict, particularly after aligning with Kiku, though they eventually reconcile. With Akira Asai, he shares a genuine friendship and connection he lacks with other classmates. His most significant relationship is with Kiku Hoshimi, whom he loves completely, ultimately choosing a shared death over life.