A boy loses his parents in a supernatural disaster known as a "Curse" and is then adopted by the Tsuchimikado family; a family that exorcises curses. Ten years later and the boy has become the head of the family and now goes about exorcising curses all the while trying to balance his job as an exorcist and his life as a student in high school.

This is the story about the daily life of a young siscon novelist, Itsuki Hashima with his perfect little sister Chihiro, the genius illustrator Nayuta Kani, his best friend Shirakawa Kyoto and some insane partners from his publishers.

Kodaka Hasegawa is just trying to get through his school life, but one day he saw the cute but forever pissed off girl Yozora Mikaduki alone, happily talking to herself. Kodaka and Yozora started discussing how to make friends, and Yozora, with her excessive action powerness started a tragic club. Then, as things went crazy, one by one other regretful cute girls also joined the club. Together they played galgames, went swimming, and they put together stage acts. Can our hero, who got suckered into the mess, become good friends with others? The even-though-it-really-needs-much-to-desired-for-but-still-very-entertaining tragic youth romance story has been born!

(Source: MangaHelpers)

A "what if" alternative version of the original story, on which the cast gets acquainted in a different manner.
The basic premise is that Kodaka, a rude looking and easily misunderstood transfer student catches a glimpse from a conversation between Sena (the most popular girl on school) and "her friend" Renka (a character from a visual novel); after a short discussion followed, Sena decides to give it a try and make some friends with Kodaka's help.
This is a tweak from the original premise, on which Kodaka catches Yozora speaking to her "air friend", and is Yozora the one who decides to start practicing how to make friends with Kodaka's help.
(Source: MangaHelpers)