Tsundere (ツンデレ?, pronounced [tsɯndeɽe]) is a Japanese character development process that describes a person who is initially cold and even hostile towards another person before gradually showing his or her warm side over time. The word is derived from the terms tsun tsun (ツンツン?), meaning to turn away in disgust, and dere dere (デレデレ?) meaning to become ‘lovey dovey’. Source: Wikipedia
I first encountered this term over at TV Tropes, which shows plenty of examples contributed by its readers.