>>712954804 (OP)
She is literally Deltarune.
What were Gaster and Alphys trying to make in Undertale? Monsters with determination.
What is Susie? A monster that acts on her own and derailed the prophecy from the start (confirmed by Gerson telling her she was the dragon and thus should have been the ch1 antagonist.)
What was Gerson trying to do in ch4? Strenghtening Susie's determination and emboldening her to ACT on her own, hell, she has several fights without Kris (Lancer, Gerson).
Gaster is nurturing Susie to become the monster with determination, she is the Deltarune, Gaster got absorbed by his creation as per UT.
(You) are an agent working alongside Gaster regardless of (you)r intentions as the player, you're stuck with Kris but this is Susie you're nurturing.
This isn't your game, this is Susie's but only Gaster knows.