Ai Momoi

Futakoi:
The oldest of the twins appearing in the series. Ai is the teacher responsible for Nozomu's, Kira's, Yura's and Keisuke's class, while Mai is the school nurse. [Wikipedia]

Futakoi Alternative:
Ai Momoi and her twin sister Mai Momoi were once classmates with Rentarō and had a relationship with him similar to his current relationship with the Shirogane twins. Since their school days, they have had a falling out, both with Rentarō and with each other.

(Source: Wikipedia)