Dandere ダンデレis a term used to described anime girls that tend to be very quite and calm due to their shyness, and exhibiting either very little or no emotions.
Actually the word Dandere ダンデレ is derived from a longer phrase: Danmari Deredere (だんまりデレデレ), where Danmari means silent and Deredere means a sweet affection towards others.
While such characters are not very common in anime series, there are a couple of Dandere that were very popular including Nagato Yuki (長門 有希) from Haruhi Suzumiya (涼宮ハルヒ) and Ayanami Rei (綾波 レイ) from Evangelion. Other dandere characters that I can think of include Primula from Shuffle! and Kiyoura Setsuna (清浦 刹那) from School Days.