AMANO Sakuya

Set in a fictional European country in 1924, a Japanese exchange student meets a mysterious, brilliant girl who only leaves the library to sleep. Her brother, a detective, relies on her mind to solve difficult mysteries, several of which draw her away from the library. (From Fate)

Ichiru Kanzaki is given an offer he can't refuse: to clear his entire debt, he has to enroll in the high school where his father works to track down his father's illegitimate daughter, his own younger half-sister, whom he has never met. But which girl is she?