This is impossible to do without massive spoilers, so probably nobody who hasn't finished the game should be reading any of this. I thought perhaps we could take a crack at unpacking some of the themes, how they're constructed, what they're trying to say, how they change as the plot progresses, etc.
It also features some very subtle and clever allusions to to other soundtracks. In addition to a ton of memorable and unique character themes, the Undertale soundtrack makes admirable use of leitmotivic development and foreshadowing and even some musical misdirection. Undertale is kind of a masterpiece, and needless to say, I was blown away by the complexity of the soundtrack.