Mysterious Girlfriend X

OVA released with the 9th volume of the manga.

Natsuo Fuji has a serious crush on one of his teachers, Hina Tachibana, but since he knows he has zero chance of ever being in a relationship with her, he lets his friends talk him into going to a party where he meets Rui. One thing leads to another and then, well, neither of them is a virgin anymore. Unfortunately, it wasn't what they expected, but that's fine. They're just ships passing in the night and they'll never have to see each other again, right? Except when Natsuo's father announces that he's getting married again, Natsuo discovers that he's getting two new step-sisters as well. Now there's a problem, because one is his teacher, Hina, and the other is Rui.

The protagonist, Naoya, is a first-year in high school. He asks out Saki, a girl he has been in love with, and she agrees to be his girlfriend. He's currently on cloud nine. But then, a beautiful girl named Nagisa comes up to Naoya one day and tells him that she wants to go out with him. Naoya feels himself wavering because Nagisa is such a good girl, and he comes to a decision! A new kind of romcom from a rather unexpected decision is about to begin!

Juuzawa Juu is a delinquent high school boy who just wants to be left alone. One day, he is approached by Ochibana Ame, who claims that she knew him in a previous life, and now wants to serve him as his "knight". At first Juu wants nothing to do with Ame, but after a classmate is murdered, he accepts her help as he looks for the killer.