Nobody expected the little, chubby, goofy, bumbling struggler to become the stalwart hero that he needed to be, did they?! That quiet kid was Neville Longbottom
Neville had one of the most noteworthy transformations in character throughout the Harry Potter series and arguably took on the biggest change in the franchise. Starting off as an apparent failure by messing up in every lesson the audience was led to feel like this kid was a purely comedic element to the series, there for us to laugh at his misfortune.
Yet, as the series progresses and insight to the young man's tragic past is revealed the comedy turns to sympathy, followed by acceptance of the kid starting to prove his worth and finally the feeling of admiration and pride watching the iconic moment of Neville standing defiant in the face of death, picking up the now vacant reigns of leadership.
Pretty hard to argue that this character deserves the title of best character arc in the series.