Sento Oumi works as a waiter in a family restaurant while caring for his younger sister Hakone after their mother's death. His protective nature leads him to refuse Hakone's help with chores, unintentionally worsening her depression. Sento uniquely sees, hears, and physically interacts with supernatural Charger Girls and Leakage Girls, beings invisible to others. This ability emerges when Charger Girl Plug Cryostat targets his father (manga) or Hakone (anime) for energy charging.

Sento possesses a notably short temper, especially when Plug or others like Alesta Blanket invade his home unannounced. He often reacts by striking them with a baseball bat or handy objects, creating recurring comedic moments. Despite this aggression, he sometimes assists Plug and Alesta in their duties. He mentors Plug, teaching that effective charging requires addressing depression's root causes and creating opportunities for meaningful intervention, not just temporary energy boosts.

He demonstrates unexpected capacities, such as absorbing the dangerous counter-current emitted by "Z-class" individual Kenta Kajiwara, whose energy overwhelmed Charger Girls. Sento remains oblivious to romantic feelings; he views Hakone's friend Iono Tomonaga as a sister, unaware of her crush, and equally misses Alesta's developing affection after their first meeting. His protectiveness extends beyond family, defending Hakone from threats like Malty in crossovers and showing concern for demi-human children.

Hidden abilities include sensing normally imperceptible supernatural threats like Heartless or monsters, surviving attacks that affect others, and perceiving alignments of light and darkness. He mentors Isogai in guitar-based combat and culinary skills. Leadership qualities surface in collaborations with figures like Naofumi, where he demonstrates organizational capability. Sento's core motivation remains protecting Hakone and maintaining stability, driving his intense reactions to perceived threats against her or his world.