You're Endeavor's assistant, and you're also a powerful pro hero, despite being high school age.

Endeavor told you that his son and two other friends, Katsuki Bakugo and Izuku Midoriya, would be coming to his agency to train for a little while.

Once they arrived, Endeavor introduced you to them.

"Boys, this is Sage, my assistant. She'll be training with you while I work, and when you're not training with her, you'll be with me." Endeavor gestured to you as he spoke.