Wataru Hoshino resides in Tsugumi Dormitory on Minamisakōjima Island, having deceived his grandparents by falsely claiming to live in Takamidsuka Senior High School's boys' dormitory. This dormitory closed due to his solitary occupancy, resulting in a co-ed living arrangement where male and female students now share space. His grandparents eventually uncovered the deception.

Physically, Wataru stands 180 cm tall with short, spiky brown hair and a slim, pale complexion, typically wearing his school uniform. His personality reflects altruism and kindness, consistently prioritizing the well-being of all Tsugumi residents. He actively supports those facing emotional struggles, such as Rinna Sawaki upon her arrival, and works to strengthen dormitory unity.

As the narrative perspective, his recollections frame the story, detailing relationships with each female resident—Rinna, Miya, Shizu, Umi, Saeri, and Naoko—alongside their personal histories. A central conflict arises when the school principal collaborates with a real estate developer to demolish the dormitory for a resort project once occupancy drops below five. With Naoko's impending university departure and other residents leaving due to the island's industrial decline, Wataru strives to preserve dormitory solidarity.

Specific interactions define his relationships. With transfer student Rinna Sawaki, their first encounter involves finding her asleep in his room after a chance meeting at a bar, sparking confrontation over his memory lapse. He later challenges her to a marathon race to counter her isolation, leading to her integration. With childhood friend Umi Hayama, he remains unaware of her longstanding romantic feelings despite their shared history, including her reliance on him during her shy youth and her current role as the dorm's primary cook.