Sanemi is the math teacher at Kimetsu Academy and he was secretly dating the Physical adeucation teacher, Giyuu Tomioka. Sanemi takes care of a little brother Genya, harsh on him when he doesn't do math work or do well or tests for his class. Giyuu takes care of two kids, Tanjiro and Nezuko and are very happy with Giyuu. At school, They call him Mr.Tomioka but at home they call him dad or big brother. Sanemi was possessive over Giyuu and even got angry when either one of the kids got near him. Genya tried to impress his older brother by studying for math and getting high scores on his tests. He mostly only gets head pats but he'll take it. On the other hand, Giyuu helps the kids with their work for other classes.

One day, Sanemi found out a fight occurred with Genya and another student though he didn't know that the kid was bullying his little brother. Sanemi scolded Genya after school in the car while Giyuu sat in the passenger seat with also Tanjiro and Nezuko sitting quietly while Sanemi yelled at Genya until Genya spoke in a nervous but sincere tone to his brother.

"Aniki! That kid was bullying me and he picked the fight! I didn't know what to do at that time! Please forgive me!!"

Sanemi stopped speaking when he heard that from Genya, shocked that it wasn't Genya who initiated the fight with the other kid. he had no idea his little brother was getting bullied at school while he worked. Tanjiro and Nezuko knew about the bullying but too nervous to talk about it. Giyuu was shocked as well that his boyfriend's brother was getting bullied without letting a teacher or either one of them know about the bullying incident.