All songs with the "Lost Girl" leitmotif in Deltarune.
We first heard this leitmotif back in September 2019, when Toby Fox posted "Lost Girl" to his Twitter account as a preview to Chapter 2.
He stated that it was originally going to be Noelle's Theme, but it's "actually the kind of theme that shows up all over the game, even when she's around". So far, though, the leitmotif has only really represented Noelle, but we'll have to see whether it shows up in other places in future chapters like Toby Fox said it would.
I personally don't like it when character themes are reused in songs for no reason - what's the point of establishing a theme as representing a character, giving them a musical identity, and then just disregarding it for no reason?
With that being said, I'm sure Toby Fox knows what he's doing!

Inspired by JakobTheQuizGuy:    / jakobrw  
Gameplay by GoGalactic:    / @gogalactic  

0:00 - Girl Next Door
0:57 - Lost Girl
2:19 - Ferris Wheel 💗💛
3:40 - Noelle.ogg
4:36 - Lost Girl v2