In a recent topic, @Xanderjedi11 brought up Noelle's (possibly canon) blogpost mentioning Raise Up Your Bat:
Coincidentally, I've recently been thinking about what this could imply about the in-universe origin of Lost Girl, so here's a bit of a crack theory.
During the Chapter 4 basement segment (my beloved), you can turn on the speaker, and it'll start playing Lost Girl. Noelle acknowledges this in the game:
* Do you... hear something...? From the basement?
Let's assume that the track itself is diegetic, and the characters hear the same thing as we do. Let's also assume that her blogpost is canon and refers to the same Raise Up Your Bat as we hear in Chapter 3.
I think the 2 songs sharing a leitmotif can mean 1 of 3 things in-universe:
Lost Girl was made first, likely by the same band, then its melody was incorporated into the chorus of Raise Up Your Bat. Noelle just happens to have a recording of Lost Girl.
Raise Up Your Bat was made first, then its chorus got extracted and developed into Lost Girl, likely by the same band.
Raise Up Your Bat was made first. Noelle liked the chorus so much that she kept listening to it on repeat. This prompted Kris to make a cover which got recorded onto a CD. Noelle eventually moved on and forgot about where she'd put the disc.
3 is the funniest option, so that's what I choose to believe.
Cool theory!
All I know about diegetic music is that Ralsei can hear the soundtrack based on the piano in chapter four, and that Rouxls also canonically just emanates his theme wherever he goes, also based on the piano in chapter four.
(Lost Girl wasn't included because I seriously considered the possiblity that what we hear isn't what the characters hear. In retrospect, that was probably silly.)
I personally think it’s made by Dess and the Raise Up Your Bat song is just either a coincidence or Dess got inspired by it. You theory still holds though and is just as likely. Also btw I think Dess would play it to Noelle to comfort her and that’s why we hear it in chapter 2 when she is alone and thinking about Dess.
I'm not sure what instrument Lost Girl plays on, but it's clearly not a piano. It doesn't matter what the instrument would look like in real life, that is simply not the right sound.
Besides, this is the same person who would stop playing and stare blankly at Noelle if she ever dared showing up. I doubt they would go out of their way to make a recording for her.
Kris is associated with pianos, not electric keyboards or whatever else. Even the church organ became an actual piano so that Kris could play Piano that may not be played that well.
If the intention was for us to realize that Lost Girl was played and recorded by Kris, then it should have been played on a piano. It wouldn't even hurt the track's somber mood.
I've heard another theory where all non-human creatures just... emit music. So all of the music is actually diagetic[1].
This is supported by the fact that when you kill everyone, the music gets slower, sadder, and more ominous.
That's only like two entities though, and nobody's sad about it. Genocide and Weird have you murdering immense amounts of the local population, drastically changing the tone of everyone's internal song.