Sansa Stark, Game of Thrones
when the series started Sansa was irritating and a foolish girl. Buta lot changed by the end of the seventh season. Sansa suffered a lot physically as well as mentally but that experience made her only stronger. She turned out to be the true ruler.
Buffy Summers, Buffy the Vampire Slayer
The cute schoolgirl Buffy met Giles who was a librarian and had close connections with the demon world. And as he thought she will be, she became a super demon fighter. She is far from perfect and has some flaws but she works to make the world a better place.