Campione
Huh? It has to be planned to happen. Has to.
Lets put it this way, why does Godou have a little sister in Campione? At this point you may say that it's because the author wanted Godou to have someone he needed to keep his secret from, or an anchor in the normal world or something like that but that isn't my question, I'm asking why a little sister?
See, in the real world we give meaning to random things because they are there but in fiction "random" things are there because they have or will have meaning. If the author just wanted there to be someone close to the main character that Godou needed or felt he needed to hide his power from he could have simply gave him a friend in school but sure a brother or sister would also work. If the author wanted there to be that conflict between normal and the all female harem he could have simply gave Godou a friend that was a girl in school his age that liked him, you know, a childhood friend type character. So, unlike the real world where having a little sister is a random event Godou having a little sister in Campione is not a random event, the author consciously gave Godou a sister and a little one at that, think about the implications of that for a second.