The Angel is (You). How that's interpreted by the characters varies on the route, and some might not even attribute it to (You), but it's undeniably (You).
Just like how it was in Undertale, where the Delta Rune prophecy speaks of an angel who's seen the surface and will cause the underground to go empty.
The actions of (You) using Frisk as a vessel determine if the underground goes empty or not, and if it does empty, how it goes about this, whether that's freedom from imprisonment or genocide leading to total erasure of the underground.
In Deltarune, the Angel will be banished to meet with its desire. This doesn't mean "the game ends" because that's stupid, it refers to Kris banishing (You) from his body once he no longer needs the SOUL in order to fulfill his promise.
Gaster assumes your desire at that point will be to reclaim your discarded vessel so that you can be truly free, meet those WONDERFUL SOMEONES for yourself, and also so that you can finish your work in HIS DELTARUNE to save the worlds.

>>509535659
>working
>class

>>280183642
>compared to the perfection that was Part 1.