Demon Queen Abyss is worried. She's not able to defeat the Hero, considered the mortal enemy of the demons. This Hero is still an inexperienced young man. He should be easy to defeat. However, the problem is that he, not knowing Abyss's true identity, considers her his “Master” and has become very fond of her. [Official Japanese](https://youngchampion.jp/series/d2ac49313ccd7)

Tomoki, a high school student lived with his older sister. What happens when a normal boy has to put up with his sadistic but popular sister?

"I" (Boku) love math. Just after the high school entrance ceremony, "I" meet a beautiful girl: Milka. Milka is a mathematical genius. She gives me many math problems, she shows me many elegant solutions. Milka and I spend a long time by discussing math in the school library. One year later, I meet another mathematical girl: Tetra. Tetra is one year younger than me, and asks me to teach her math. While I teach her, she begins to understand math and to love its elegance gradually. In the second volume (series), we talk about Pythagoras Theorem, Elementary Number Theory, Group, Ring, Field, and Fermat's Last Theorem.