New Neville

For me, the character who has really undergone the most change throughout the series so far would have to be Neville Longbottom. When we were first introduced to Neville, he was more or less an idiot who couldn't remember a thing to save his life. His biggest fear was his grandmother. Pathetic. But then, all of a sudden, in the fifth book we see Neville develop some gusto about himself. Harry constantly notices how well Neville picks up spells and performs them well compared to the other members of Dumbledore's Army. One would expect the old Neville to shy away from going to the Department of Mysteries, but Neville 2.0 is more than willing to accompany Harry there, even with the prospect of facing Dark wizards and maybe even Voldemort. He even remains steadfast in helping Harry save the others after his wand has been broken in half and his mouth and nose bloodied and broken.
A case could also be made for Ginny. In the first and second books, she is mostly shy around Harry and isn't really seen as an important character (accept for the whole diary bit).  I think we finally see her, as with Neville, come into her own in the fifth book. She becomes a member of the Quidditch team and is relatively good at it and she also goes to the Department of Mysteries to help Harry. She seems to have an unwavering dedication to Harry...wonder where that could end up going?