So her family name starts with S and her given name starts with Y (Because she confirms that her real name's initials are reverse of her stage name's) and it "feels like a prototype of her stage name," which probably suggests that its similar in ways other than the initials. So from "Yoruzaki Sasa," the given name is kind of nonsense, but the family name can easily be split into "Yoru-saki," so maybe "Saki-something Yoru-something?" It's also meant to be "ordinary," so maybe the family name is, like, "Sakiyama" or "Sakimori" or something?