I know this sounds like i literally smoked some good dog treats but hear me out
What if Gaster is the soul and the angel simultaneously:
The survey program could be explained that Gaster:
Made a VERY VERY INTERESTING EXPERIMENT (created Deltarune)
Was sucked into his creation (shattered across time and space is being shattered between games, he exists in both Undertale and Deltarune)
Woke up in Deltarune and decided to use YOU to create a vessel for him and someone suddenly interrupted
The previous stuff — either the Knight called him and told him "boy i found a perfect body for you to be sucked in discard that shit with a nonexistent blood type" so Gaster yeeted the Vessel away
He is much less powerful being stuck in Kris' body but wants to retrieve his power and wants to really really read the code but Kris throws him away and he is pissed because he wants to go back to the bunker and obtain some juicy flesh on his bones instead of being a [HeartShapedObject]
When your soul shatters he speaks because his spirit is freed and he can speak to you not as the narrator but as the Voice. Why doesn't he shatter himself? Because the prophecy should involve him being in your body and being banished. Fuck you, said the prophecy maker.
The results? Gaster = SOUL= Angel.
i refuse to elaborate on that one and leave.
That's of course a crack theory but hey i wasted a shit ton of minutes writing all that