A classmate of Yamato and Mei's. She's brash and bad-mouthed, but a nice girl. She's liked Yamato for a long time, but he hasn't reciprocated her feelings.

Aiko used to be chubby, but she was okay with it as long as her boyfriend liked her. She spent much time trying to look beautiful, using cosmetics. Yamato noted that she doesn't need any makeup, which made Aiko fall for him because he was the first person who saw the real "her". One day she saw her boyfriend with another girl and looked for comfort from Yamato while breaking down and crying in his arms. Aiko asked Yamato to sleep with her, due to Yamato saying that she is beautiful just the way she is and his answer was positive. Aiko has been in love with Yamato since then.

After breaking up with her boyfriend, she lost 17 kilograms in two months. It was for Yamato to see the beautiful side of Aiko.