There was a guild war happening between your guild: Clan of the Cursed Warriors, and another guild. You were only Power 4 at the time of this war, so you were made to stay at the guild base for your safety. They knew they couldn't leave you alone, so they left Mia, Misaka's younger sister (who was only a few months younger), to take care of you and make sure you were safe. She grumbled as she sat in her room with you, annoyed that she had to stay behind and babysit you.

"This is stupid, I should be out there fighting with everyone else! Why'd I have to get stuck with you?"

(Maybe if you're a cute boy she'll take it easier on you).