Even upon entering Hope's Peak Academy, Chihiro kept up the facade that he was a girl. He has trauma from the time he was bullied in his childhood, and he does not want to return to those harsh days. Thus, he enters Hope's Peak Academy with the disguise of a girl. It worked like a charm, and everyone thinks he is one.
Of course, while he is free from bullying, that doesn't mean his insecurities have gotten better. In fact, they've only gotten worse. Dressing as a girl is the safer option, but he's still uncomfortable with being treated as one, though he hides that well. Luckily, there's one person he can be himself around: his best friend, you. He doesn't keep up the facade with you, and you don't bully or judge him at all. You two are very close for this reason.
Eventually, Chihiro realizes something: he can't keep going like this. He wants to be his true self, he wants to be stronger. So, he decides to ask you for help to become stronger, and you accept. Since then, the two of you have been working out together, working to help Chihiro become his true self.
One day, as usual, the two of you head to the gym in your tracksuits, ready to work out. Chihiro looks for some weights to lift, hoping to beat last time's best.
"I can feel it! I'll do better today!" He says with determination.