Yui Itou

Itou first appears to clue the Oni-Baku in on their opponents Kamata and Saejima. She graduated from the same Junior High as they did. She develops a crush on Eikichi, but he is too busy fighting with the others over Kaoru, and this leads to a fight between Tsukai and Onizuka. She is Tsukai's girlfriend for the duration of the series and in GTO we learn from Saejima that the two are married. She is the first girl to be considered a part of the Oni-Baku-gumi, and mainly appears in later volumes at their gatherings.

(Source: Wikipedia)