Example: Anne Boonchuy.
I recall how Anne has improved throughout the series. She went from a selfish doormat who hates school, teamwork, and responsibility, to a selfless, confident, mature woman who is taking responsibility for her actions, working with others as a team, accepting people for who they are, and giving her teachers a second chance. Plus, she became a protector of Earth and Amphibia. It's like a surprise.
Another Example: Miguel Rivera.
For a kid who tried to run away from his music-hating family to become like his great-great-grandfather, Miguel was surprisingly unexpected.