Quote from eccentrrick
One of my bestfriends was a tsundere. She was really cold to me at first, and told me I was annoying, but I kept trying to get to know her, and she was genuinely one of the nicest person ever! *o*; It felt like I just got some rare gem in my hands! XD The feeling of discovering the dere-ness was really nice!
That girl sounds like a Kuudere, not a tsundere.
Kuudere = cold and indifferent on the outside, nice on the inside.
Tsundere = harsh, scathing and rude on the outside, sweet on the inside.
I like kuuderes. They're the only type of 'something-dere' I like to see in manga / anime.