Asuka Soryuu Langley from Neon Genesis Evangelion
To be fair, I never hated her, but I did think she was kind of annoying when she first appeared. Especially since she looked like another stereotypical tsundere character. But as the show went on and we learned more about her and got to understand her behavior better, the more I came to like her. And now she's my favorite character on the show (together with Kaworu). Which is why I was so annoyed that they gave her big fight scene in the second new EVA movie to Mari *blech*.