Miki

Miki and Yuudai used to date, or something similar, since neither is certain if they were ever officially a couple.
Mike is very fashionable and immodest. She has long nails and wears makeup all the time. She shows up at Yuudai's apartment after fighting with her boyfriend (Ken) and invites herself to stay the night.
She makes herself very comfortable, and when Chizuru comes to Yuudai's the next day to apologize, she doesn't even think of hiding.
She later makes up with her boyfriend.