He has little self-confidence, despite the encouragement of Fat Gum and Mirio Togata. He's part of the big 3 and it's not just because of his powerful quirk but because of his technique. In fact, the strict and devaluing view he takes of himself pushes him to always surpass himself and not rest on his laurels. So when he pushes aside his sickly shyness to plunge into combat, he's transformed and can handle several villains on his own.