Serizawa Aiko

She was a girl from Sakamoto and Oda's class. Sakamoto had a crush on her, but Oda seduced and took her from him. Aiko only appeared in the story in Sakamoto's flashback in high school. There, she was the object of Sakamoto's affection. Even though he was in love with her, he didn't have the courage to confess to her yet. After Sakamoto told Oda about his feelings for Aiko, Oda insulted him being too naive and an idiot. Oda explains that if Sakamoto is too passive, Aiko would be with someone else. Later on, Oda had sex with Aiko after she has confessed to Oda. When her friends and Oda's girlfriend found out about it, she burst into tears in guilt and Sakamoto attacked Oda, partially because of Aiko. She hasn't been seen since. (Source: Btooom! Wikia)