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When the Vessel is created by the player and Gaster that person who sounds highly similar to a certain scientist but not proved to be him, both of them grant them a soul, the player's soul. However, that soul ends up being ripped away from the Vessel to get into Kris - all of this happens exactly like in the game.
After being created and discarded, the Vessel grows frustrated at their current situation. Once they somehow end up in the Light World and thus gain access to the Dark World like Kris and Susie, they decided to look around everywhere for their soul, as well as traces of the people who created them.
They would meet Jevil, then Spamton, after hearing of their stories and suspecting they may be related to the scientist at the origin of their creation. They'd eventually get the two of them to join their team, one after the other. The three would make an alliance and swear that they'd attain freedom together.
Maybe there could be a final confrontation between the Vesssel and Kris - the Vessel, desperately trying to get their soul back, not understanding what it entails ; and Kris, knowing they have no choice but to keep it, sadly, in order to survive.
Spamton and Jevil's relationship could also be a main focus of the story. The two of them have a very different vision of 'freedom', after all. I imagine Jevil would intentionally push Spamton's buttons (reminding him that he is a puppet, that he will never cut his strings) and causing him all sorts of existential dread crises in order to bring him to the same state of madness he is in, because, for Jevil, the only way to be truly happy is his way. That would cause tensions within the group, with Jevil truly meaning well but not understanding that Spamton and him are not the same, and their problems will not be solved the same way - and Spamton, failing to communicate his feelings and simply avoiding the jester like the plague instead of confronting him.
I based this off of a dream I had where deltarune chapter 3 came out. When I booted it up, kris was standing in a town full of vessels that spoke cryptic messages in latin. I couldn’t get it out of my head, so I decided to draw it.
Undertale and Deltarune has got me by the jugular again. This is how I cope. 👍✨