It's been a while since I watched Happy Feet, so I apologize if this gets vague.
Mumble is humble, kind, but confused about himself. He becomes skittish because of how he is demonized for dancing, and I do think that carries on into adulthood at the zoo. When he finds himself there, though, he becomes a lot more open - but never really losing that sense of quiet humbleness, just starting to grow out of his self doubt. That's how I'd describe him, even in just the first movie.
That's the most I can recall. Hope it helps!
