Ever since Chapters 3 and 4 came out Ive changed my stance on Gasters involvement in Deltarune.

Before that I was certain he was going to be the main villain [theres still a chance].

But now he seems like a strange onlooker of all the events.

My personal theory is he introduced us [the player/soul] into the world to see if we could change the prophecy or to see if freedom could be achieved in a predetermined world.