Yuki Yuna is a Hero Season 1 Episode 3

Moral Poise

A month following their last encounter, the girls prepare to face off against the fifth Vertex, only for another hero named Karin Miyoshi to suddenly appear and defeat it single-handedly. The next day, Karin transfers into Yuna and Mimori's class, claiming herself to be a fully trained hero sent to supervise the newbies. She reluctantly joins the Hero Club, though is stubborn about becoming friends with the others. That weekend, as the girls plan to meet somewhere, Karin skips out on it, feeling she should not be led astray from her duty as a hero. Later that night, the girls show up at Karin's apartment, revealing they had planned to surprise her with a birthday party. At the end of the day, Karin is starting to get along with them.