probably yves.

Maybe its her soul emitting some kind of melody (melody probably related to the "Lost girl" one). Plus, Onionsan said something about a band. In chapter 3, there's this mini game, where the lighteners are a band and the song that they're playing has some similar notes to "Lost Girl".

That's one theory, the other one is that there might a connection (a tunnel maybe) between the lake where Onionsan is, and the shelter. This theory doesn't really convince me but, that pond is also really close to Asgore's flower shop, and I feel like he's more related to all the knight-thing (kinda like Carol) than we think, since he mentioned something about the black shard in chapter 3, so I think that's curious.

sorry but this video is like a parasitic species to me