SO, everybody is always talking about which theme belongs to these two characters, and i think i might have figured it out. through the whole game, there is really only one theme that sticks with us from beginning to close. most people attribute it to Asriel, but remember Asriel doesn't appear til the very end, and we hear this theme before we see ANY characters. Where?

https://www.youtube.com/watch?v=kRT9vY2f7tg

The opening menu itself, before any character joins it, is hopes and dreams. This is before even Flowey is introduced to us. Hopes and dreams belongs to whoever is falling into the underground, which would be frisk. but why do i also say it's Chara's theme?

https://www.youtube.com/watch?v=hRnFz-a7jWg

After a true pacifist run, this is what you hear when Flowey speaks to Chara, after Frisk has been separated from them. Flowey is not talking to US the player but Chara, Even acknoledging that before he leaves. It's the start menu theme (empty again) but super slow.

SO, I would wager that this motif Doesn't belong to anyone but the partners of peace (or the partners in crime). In fact, the Final battle with Asriel clinches this as what can these two do but "Hope and Dream".