Yōta Oshimoto is the younger brother of Kameido High tennis club member Yuri Oshimoto. He leads a relatively ordinary life, a stark contrast to the eccentric characters around him. During summer break, he serves as a butler for wealthy tennis club member Nasuno Takamiya at her family's vacation cottage, as depicted in the spin-off series.

In this role, Yōta routinely carries out trivial or absurd tasks dictated by Nasuno, frequently under unreasonable conditions. He consistently acts as the straight man, responding with practical, confused, or exasperated reactions to the bizarre behavior of Nasuno and her family. His responsibilities involve managing situations where Nasuno's family exhibits a lack of reason or restraint, establishing him as the primary voice of stability and practicality within their unconventional environment.

Yōta is middle-school-aged during these events, further highlighting the contrast with the older, more erratic characters he serves.