WMG / Doki Doki Literature Girls

The new girl is actually an amalgamation of all the girls.

This idea is actually pretty similar to Bernkastel's origins in Umineko: When They Cry. In the games, it is revealed that Bernkastel is the result of all the Rika Furudes that had died in several different timelines. As such, the new girl that Sayori meets in the recent story arc could actually be all four girls — or one of them — merging into one after all of the many deaths they suffered from the original game.