"Rental OniiChan" centers around the concept of a rental service where individuals can hire an older brother figure to fulfill various roles, such as providing companionship, advice, or emotional support. The protagonist, Haruto, is a college student who works for this service, offering his services as a "rental brother" to clients in need. Each chapter typically explores a different client's story, showcasing their unique struggles and how Haruto steps in to help, often blurring the lines between his professional role and genuine human connection.

The narrative delves into the lives of Haruto's clients, who range from lonely individuals seeking familial warmth to those grappling with personal crises. Through these interactions, Haruto's own character is gradually revealed, including his motivations for taking on this unconventional job and his internal conflicts about maintaining professional boundaries while forming emotional bonds. The series balances lighthearted moments with deeper emotional undertones, addressing themes of loneliness, family dynamics, and the search for belonging.

Supporting characters include Haruto's coworkers and recurring clients, each contributing to the overarching exploration of human relationships. The episodic structure allows for a diverse range of stories, from heartwarming to bittersweet, while maintaining a cohesive thread through Haruto's evolving perspective on his role and the impact he has on others. The manga avoids heavy drama, instead opting for a slice-of-life approach that emphasizes relatable, everyday struggles and the small but meaningful ways people can support one another.