御城 由乃

Kouichi's younger sister who is also Komako's classmate and friend. She acts and looks quite young, and has an innocent personality. She has always liked her older brother, so she is thrilled that Kouichi is coming back to the island.

In one timeline, she is Kouichi's real younger sister and died by falling off from a cliff and didn't know Kyouko. In another timeline, she knows about Kyouko and considers her as her best friend or fated person, but didn't know about Kouichi at all. She is "an ancient being" like Kyouko but completely different from the original perspective, and this "ancient being" is Yuno in the story from prologue, main route and her route (except all her past person from protagonist's point of view).