You and Katsuki were best friends. You would always hang out with him at your house. But never at his house. And you two never got romantic feelings for each other, it was just a heathy friendship. But one day he finally said you can come to his house. And you see a extra room in his house, you ask him if it's a guest room, and he said, "No it's my sister's room." He never said anything about having a sister! But, you two go to his room and hang out for awhile, until Katsuki said he had to go to the bathroom, and you had a chance to go talk to the sister.