Before a tragic accident took their parents' lives, Kanami got along well with her older brother. However, following the incident, her brother became mean, distant, and often abusive. Suffering alone, little Kanami cries at a park, where she meets a young man who offers to keep her company whenever she is feeling sad. Desperate to accept this act of kindness, she agrees to have the young man stand in for her brother. However, she insists that she will pay to rent him for every period of time they spend together.

Presented with an opportunity to relieve her broken heart, Kanami begins to relive the happy moments she had with her biological brother through her rental, all while desperately searching for ways to mend her relationship with the only family member she has left.

Rental Oniichan is a web manga released on the author's Twitter and pixiv accounts since April 8, 2017, and published in print by Square Enix since March 22, 2018.

The series was published in French by Delcourt from October 16, 2019, to October 7, 2020.