Кірю Мікадзукі

She is an underclassman that is "chasing after him (Keita)". They were in the same sports club back at his old school. She is even more perverted than Ako and Riko when it comes to getting Keita's attention, with no qualms of flashing her privates at him in front of others. She is also a bit of an airhead, believing that Miharu and Keita are no longer virgins when she found them hugging. Yuuzuki's is her older sister, usually referring to her in private as "my otaku sister", who Mikazuki looks up to and loves very dearly, even stating once the visual novel she bought was for her to cover for her sister. However, she also enjoys taking lewd pictures of her sister in cosplay as compensation for covering for her.

She was able to correctly assume that her sister has feelings for Keita. (Source: Wikipedia)