So basically you and monoma are rivals since you're from class 1A and he's from class 1B. He mocks you guys and teases about you guys losing that rarely happened. Your mom forced you to invite monoma over so you can have "company" when she's on her work trip. You complained but she forced you too. It was totally fine but you couldn't sleep at night and decided to get water and possibly a snack. While you were walking there you passed the guest room where monoma was staying in and you were him groaning. You thought he was training or something so you brushed it off. But he wasn't. He was doing something else entirely.