lost girl feels like the his theme of deltarune
Hmmm, feels like an apt comparison on some level. But also, it's important to remember...
Part of the power of "His Theme" is the way it's saved up to the very end of the game.
Like, we hear "Memory", the music-box version of the theme, a few times before, and it's remixed into several other tracks like "Undertale" and "Finale", but it's only during the emotional climax of the True Pacifist Route that the game reveals the 'actual' Theme.
It's a deliberate inverse to how most Leitmotifs and Themes are used throughout the game. Usually, a theme is introduced on it's own standalone track, and then later important moments are punctuated with farther remixes and references to this track. For example, Alphys' theme plays as a Player first meets Alphys, and later on we get to hear a piano rendition ("She's Playing Piano") and a darker remix of it ("Here We Are").
While one could argue that Asriel's 'basic theme' is actually "Memory" and that "His Theme" is just the orchestral variation of it, I think the song's title leaves little room for doubt as for the intent. This is Asriel's theme, his theme. After hearing little hints and references to it throughout the game, this is when the Player finally gets to actually hear the 'true/original version'. Just like Asriel has been affecting the story since the very beginning, but it's only now that the Player begins to truly understand him, and peel away the layers of villainous posturing to see the real Asriel underneath.
This is such an important part of what makes "His Theme" work so well, that I feel like this is kinda required for anything to be the "His Theme of Deltarune". That if "Lost Girl" is any kind of counterpart to it, that it's less of a direct comparison and more that "Lost Girl" is a remix of a theme we haven't quite gotten to fully hear just yet.
... Okay wait I just realized that by my own logic you can argue the real "His Theme" counterpart in Deltarune is "Feature Presentation" with "HEY EVERY !" being it's 'Memory"