Kōshi Nagumo is a junior high school student rendered homeless when a fire destroys his family's residence. His father abandons him due to financial strain, leaving him penniless and living on the streets until he collapses from hunger. Mineru Wachi discovers him in this state and brings him to the Goddess Dormitory at Seikan Women's University, offering him shelter and food. Initially hesitant to become dorm mother due to resident Atena Saotome's androphobia, Kōshi declines to avoid making her uncomfortable. Atena later rescues him from freezing outdoors and accepts him into the dormitory, marking the start of his new life there.

Physically, Kōshi appears as a young boy with short dark-brown hair, light-brown eyes, and an "adorable" demeanor. He typically wears a standard school uniform but is occasionally forced into cross-dressing or cosplay by the dorm residents, causing him significant embarrassment.

Personality-wise, Kōshi displays maturity and responsibility beyond his years, stemming from his traumatic abandonment. He suppresses his true feelings, particularly fears of homelessness and losing his surrogate family, to maintain stability. As dorm mother, he diligently performs chores and scolds the residents for irresponsible behavior, like Mineru's hazardous experiments or Serene Hozumi's unhygienic habits. Though easily flustered by the older residents' overt sexuality, he exhibits strong willpower in resisting seduction and openly warns them to be mindful of their actions. His empathy is central; he counsels residents through insecurities and defends them against external criticism. He avoids romantic entanglements due to a deep-seated fear of jeopardizing his home and relationships. He also harbors insecurity about his age, feeling compelled to act mature and embarrassed by childish behavior.

His daily life involves managing the dormitory's chaotic environment. Tasks include cooking, cleaning, and mediating conflicts among residents, such as Frey's coercive cosplay sessions or Kiriya Senshō's destructive stress relief. Key events include nearly drowning while failing to save Sutea Kouroya (highlighting his inability to swim), collapsing from overwork and repressed trauma, and enduring a public sexual assault attempt after exposure to Mineru's aphrodisiac pheromones. His character development shows gradual emotional opening, notably subconscious jealousy toward Atena during Valentine's Day, indicating budding romantic feelings.

Relationships with the dorm residents drive much of his growth. Atena Saotome evolves from fearing him to developing romantic and sisterly feelings; Kōshi initially views her as a protective older sister before reciprocating her affection. Mineru Wachi earns his gratitude and respect for rescuing him, though her scientific mishaps and sexuality unsettle him. Kiriya Senshō bonds with him over shared vulnerability, leading to her romantic confession (which he rejects due to his feelings for Atena). Frey provides unique emotional support, helping him confront fears through cosplay-themed roleplay. Serene Hozumi unsettles him with erratic behavior and secretive kisses but benefits from his kindness in mending her belongings. Sutea Kouroya, his childhood friend, relies on him for comfort due to her heat sensitivity, though her tsundere tendencies result in violent outbursts. His relationship with his father remains fraught with resentment until a genuine reconciliation occurs.

Kōshi's journey emphasizes resilience amid adversity, balancing responsibility with emotional vulnerability while navigating the complexities of found family.