Renta: This is the start to a spin-off based on the popular series, "I May Be Your Big Brother, But I Want You To Be Sweet On Me," which focuses on Mr. Mizuhara and Ayumu Yuzuki! Mr. Mizuhara is a high school teacher who was turned down by Kanata, who used to work at "Little Brother * Cafe Frankfurt."
He's super jealous that Kanata is with Keita now, but since he's a serious teacher, he focuses on coaching the swim team to move on with his life.

One day, Kanata's friend Ayumu, the liveliest member of the swim team, starts acting rebellious. Ayumu is usually friendly and talkative, so Mr. Mizuhara approaches him out of worry...
Main story: I May Be Your Big Brother But I Want You To Be Sweet On Me