Arimi is a former classmate of Yuu's at his previous high school. She convinces Yuu to be her "boyfriend" for three months, during which they act as a couple on a trial basis. Arimi attempts to make Yuu to fall in love with her during this time, but fails. At the end of the three months, he still thought of her as a friend.
Miki's indecisiveness about Ginta and Yuu frustrates her, as she does not think that Yuu deserves someone who cannot commit to one person. Arimi teams up with Ginta in an attempt to drive Yuu and Miki apart from each other. However, she eventually realizes that Yuu's behavior around Miki indicates that he does like her, so she stops her pursuit of him. Some time later, she develops feelings for Ginta and they become a couple.