The primary setting for the story is on the campus of a private all-girl school with Christian undertones, such as a large cross visible on the front of the school's main building. Seio was founded in 1886 by Mizuho's ancestor; the school's motto is "mercy and forgiveness".

Incidentally, the name of the school the characters attend was Keisen Girls School (恵泉女学院, Keisen Jogakuin?) at first. This was used in original PC version, the drama CD of the visual novel and for the Caramel Box Yarukibako fan disc. This name was later ascertained to be similar to a name of an actual school, which led to the school's name officially changing to Seio Girls School (聖應女学院, Seiō Jogakuin?, 聖応女学院 as written in PlayStation 2 version).[3] Since then, Seio is used in PS2 version, DVD edition, Caramel Box Yarukibako Fukkoku-ban and anime series.