Chiaki Minami

Chiaki is the youngest sister in elementary school. She is in class 5-2 (a reference to Kyo no Gononi). Her apparent high intelligence often makes her overanalyse simple stuff. She also tends to ignore idiotic behaviour displayed by Kana and Makoto. Even being the youngest, she is the meanest out of the three; most of the time, she is quiet during group activities, and almost always uses the word "bakayarō", which is a more forceful form for "idiot" in Japanese ("baka" being the less forceful term).

(Source: Wikipedia)