Niina Isokawa

Nīna is a blond-haired girl who goes to the same class as Marie. She has an American mother, a Japanese father, and a great deal of energy. She's very busty for her young age and is physically very active and extremely friendly, quickly attaching herself to Marie to the latter's initial displeasure. In the manga she has a crush on Yuji (who is torn between her being underage and her ample bust) and often refers to him as onii-chan.

(Source: Wikipedia)