[chronotrig]

Well, I think we should remember all of the scenes hinting about Beatrice. There was a fairly huge portion of EP4 describing Maria's and Ange's lives, saying that they were 'witches'. And then, we hear Ange say that the only way to understand Beatrice is to understand witches. Reading through that section again, it seems to me that it's a pretty direct hint that "Beatrice" is the same sort of kid as Maria and Ange. i.e. extremely introverted, lonely, bullied a lot, creates lots of imaginary friends to pass time, and wants that imaginary world to be acknowledged by those around them (though most strongly in Maria's case). Since we also know that someone was playing witches with Maria a lot, and it's hard to imagine an ulterior motive for that, there's plenty of evidence that a person matching this description lives on the island.

Also, everyone knows about the evidence that Beatrice might have been a servant, and that both Shannon and Kanon were likely candidates (especially because we see them meeting Beatrice in EP2, which is the same sort of 'meeting' that Ange has with MARIA and the stakes). Plus, if we follow the definition of 'witch' that applies to Ange and Maria, Beatrice is almost certainly either a kid or a child-like person.

Just put those two things together, imagine a servant living a life similar to Maria's, and I think that gives you Yasu pretty clearly. And all based on evidence from EP4 and before.