Fumio Usui is the mother of Kenta Usui, the main male protagonist of the story. She is a young single parent, having given birth to Kenta when she was only sixteen years old. Because of her youth, she is frequently mistaken for Kenta's older sister rather than his mother. Fumio has shoulder-length dark hair and is typically dressed in casual clothes like t-shirts and skirts.

Fumio’s life has been shaped by a series of profound hardships. She became pregnant by her high school boyfriend, Shuusei Iizuka. Her mother was furious about the pregnancy and demanded she get an abortion, a request Fumio refused. During her pregnancy, she endured both verbal and physical abuse from her mother. After Kenta was born, her mother ignored the child, speaking to him only to call him a disgrace. To protect her son from this toxic environment, Fumio fled her hometown with Kenta to start a new life in Shiihaba City. Despite this painful history, Fumio holds no regret for her choices and is deeply grateful to have Kenta in her life. She later demonstrates her capacity for forgiveness and familial duty by returning to take care of her mother when she became seriously ill.

In her daily life, Fumio struggles with depression. This stems largely from her inability to maintain steady employment. It is speculated by the vampire Anju Maaka that Fumio emits overactive pheromones, which make her powerfully attractive to men. This often leads to male co-workers sexually harassing her, a situation that typically results in her being fired rather than the perpetrators. This recurring pattern has left her with a generally dejected and melancholic demeanor. When she is able to find a job in a more professional environment without such harassment, she becomes noticeably more cheerful.

Her role in the story is primarily as a supporting character, but she is the catalyst for a major turning point in the plot. Early in the series, the vampire Karin Maaka bites Fumio and injects her with blood. Instead of falling ill, Fumio undergoes a dramatic and immediate transformation. She becomes cheerful, energetic, and full of self-confidence. It is later speculated that Karin’s blood has the power to make people become who they truly want to be. This sudden and inexplicable change in his mother’s personality, combined with the strange bite marks on her neck, is what first makes Kenta intensely suspicious of Karin, ultimately leading him to discover her true nature as a vampire.

Fumio’s most significant relationship is with her son, Kenta. Despite the challenges of poverty and living together in a cramped one-room apartment, they share a close bond. She has a playful, if sometimes annoying, habit of telling Kenta on every leap year that he is now a year older. When Kenta announces his intention to marry Karin, Fumio’s first reaction is not anger but shock, as she assumes the two are getting married because Karin is pregnant. This prompts her to awkwardly ask Karin about her due date, horrifying and embarrassing the young vampire. This reaction reveals Fumio’s deep-seated concern that her son might repeat the mistake she made in her own youth.

Throughout the narrative, Fumio shows subtle development. While she begins as a depressed victim of her circumstances, the immediate effects of Karin’s blood give a glimpse of the confident woman she could be. More permanently, she shows emotional resilience by reconciling with her past, caring for her abusive mother, and eventually finding more stable employment. Her character arc moves from one of sorrow and struggle to a more stable, and at times humorously embarrassing, maternal figure who is immensely happy that her son has found love with someone like Karin.