Banbi

Bambi is a "Beastkin" who can transform into a bear when angry. He restrained himself so that he wouldn't transform, but then he reunites with Ookuma, who first made him realize he was a Beastkin?! Getting angry and sad when he was manipulated by Ookuma... the emotions that he had held back overflowed.

"I hate you!"

That's what he thought... but when he saw Ookuma laughing while heartbroken, his emotions welled up, and he turned into a bear?!

(Source: Nihon Bungeisha, translated)