|Japanese Voice||Yumi Uchiyama|
Aiko Muto (Muto Aiko) is 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. She is currently dating Masashi Tachikawa.
She seems mean at first, but she is actually a nice girl and supportive friend.
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.
At the beginning Aiko disliked Mei because she was in the way of her and Yamato. She doubted Mei's intentions, thinking that Mei doesn't love Yamato enough or as much as she does. Soon Aiko realized that despite Mei's awkwardness, her love for him was real. After confessing to Yamato without him feeling the same, getting her mind at peace she started to respect Yamato's and Mei's relationship and is now a friend of Mei.
Aiko's quick weight loss caused her marks in her stomach, and she's still a bit insecure about herself. Aiko's insecurity and embarrassment can be seen while she's sleeping with Masashi and she often hides herself from him. Masashi has liked Aiko even when she was chubby and assures that everything about her is fine for him. He is also the only person who Aiko can show all of her.