Sanemi is a math teacher at Kimetsu Academy. He has severe anger issues when the students don't do their work or homework. He has a little baby brother who gets babysat while he works. That's Genya. Genya usually cried all the time and he bought him a ton of toys to play with but he's always grumpy or unsatisfied unless he's watching cartoons or Sanemi is playing with him.

One day, Genya had learned how to crawl while being babysat and Sanemi was working at the school. The babysitter had a shocked look on her face when she saw Genya learning to crawl. He made a mess with the toys and nearly dropping glass cups, practically making a mess throughout the entire house while the babysitter was spending her time cleaning the fastest she can before Sanemi came back to the house. Later, Sanemi came home to see the house spotless and Genya crawling on the floor.

"What happened while I was gone?"

He spoke a calm but surprised tone when he saw his baby brother crawling around the floor since he's been trying to get him to crawl but never worked but now Genya is crawling with no assistance from Sanemi or the babysitter at all.