From Perfect Illusions:

The guy who helped Mari, could he be Mari's ideal man?! Mari is super excited to have finally found her Prince Charming, but this guy, Chiaki, unfortunately has a dual personality! Due to some family circumstances, they start living together, when his mischievous personality appears and unfortunately snatches Mari's first kiss! Who is that man? What has she given him?!

Vol.01 Extra Story: Proclamation of Love
Chiharu has always loved Shinji... But they're cousins, it could never work out, could it?