Okay do well have one song, and there will be sad and happy and instrumental versions of that song. And we'll use that song for everything requiring music. Except for this one optional boss battle that is the result of specific choices that were made. He gets a super dope completely different song that he sings himself.
It's called leitmotifs, and Toby Fox is a master of it. You introduce a character and their theme so that a person listening connects the two. And then you can bring up those themes in other songs, and without any context, people can know who those songs relate to and even guess what's happening.