Galatea

A mermaid in a well that showed up in Rosetta's backyard one day.
For some reason, she really wants Rosetta to become friends with her.

Galatea as a mermaid isn't actually real. The mermaid is actually a vessel Pygmalion made to try and lure Rosetta to being consumed by him, as he had done with the actual Galatea.

Her last name is Andilet.