Arimi Suzuki

"A former classmate of Yuu's at his previous high school. She convinces Yuu to have her become his "girlfriend" for three months, during which they act as a couple on a trial basis of sorts. Arimi attempts to get Yuu to fall in love with her during this time, but fails, as at the end of the three months he still thought of her as a friend. Arimi thus knows Yuu more than Miki does.

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 Miki's suitor 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."

(Source: Wikipedia)