Tsundere (ツンデレ)

Tsundere (ツンデレ) is a Japanese word for a character archetype which describes a person with a conceited, spiky, combative personality that suddenly becomes modest and loving when triggered by some sort of cause (such as being alone with someone). It can also describe a personality that is good-willed, but its attitude and actions often contradict its nature. Originally found in Japanese bishōjo games, the word is now part of the otaku moe phenomenon, reaching into other media such as anime, manga, novels, and even mass media.

# Reference
http://en.wikipedia.org/wiki/Tsundere