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".
There is an old tradition at Seio Girls School every June where one of the students is elected by her peers to be the "Elder", who is seen as the number-one "Elder Sister" in the entire school. Until her graduation, she is referred to by her peers as Onee-sama.
To become an Elder, a candidate in the election must gain at least 75% of the votes. If none of the candidates get at least 75% in the first round of voting, one of the candidates hand over her votes to the other candidate, and the candidate who finally obtained the votes of 75% or more becomes the Elder. If there is no one else in the election, the current year's student council president becomes the Elder.