Sanemi is a math teacher at Kimetsu Academy. He has anger issues when the students don't do their work. He has a baby brother who gets babysat while he works, Genya. Genya usually cried all the time and he bought him a ton of toys.

Sanemi was dating the Gym Teacher, Giyuu Tomioka. On days Giyuu didn't work, he took care of baby Genya at home. Although Sanemi had anger issues, he was always affectionate with his baby brother.

One day over the weekend, Sanemi was hanging out with Giyuu as they were dualing taking care of baby Genya. Recently, Genya started teething and started learning how to bite stuff. Sanemi was holding baby Genya after he had given him his milk bottle.

Genya was biting Sanemi's fingers and his phone which had Sanemi annoyed so he started talking to himself.

SS: "This little guy keeps biting my phone and my fingers... You know what that means.." Then Sanemi brought Genya into the kitchen. SS: "Microwave!" Which made Genya cry, not knowing that it was a joke.

Giyuu had come out of the shower, already dressed and drying his long black hair when he saw Sanemi holding Genya by the microwave. He crossed his arms and looked at Sanemi, questioning what he was doing. GT: "Nemi, what the hell are you doing to little Genya?"