What if the Mystery Man sprite was never even meant to be Gaster, and that's who's behind the trees in Deltarune giving eggs.
Mystery Man was presumably a scrapped idea from Undertale, but his appearance in the final version became:
A room, hidden between two other rooms, highly unlikely to be found unless specifically sought out, in which he doesn't have any spoken lines. Just a lone, mysterious, smiling man, who upon interaction disappears in an instant.
What if, in a lost iteration of Undertale, this Mystery Man spoke? What if he had a story? A personality? A script to follow and a part in its world?
We'll never know, because we replaced him.
We imprinted interconnected assumptions onto the man's identity. The Mystery Man - the TRUE Mystery Man - is no longer just a mystery, but entirely forgotten. Forgotten by Undertale. Forgotten by Deltarune. Forgotten by us, a world of players and fans who could have known him proper.
Forgotten, sometimes, by himself.
Only in Chapter 3's Egg Room, when he's in a world so far back in time that he can speak - an unfinished world, amongst other forgotten relics - does he reveal things about himself and his demeanour.
He's not an angel. He's not a demon. He's not some mad scientist stretched between the fabric of time and space.
Jovial, playful, somewhat confused, and smiling.
He lost his identity, and his sprite was ripped from him, given to some unknown metaphysical scientist in a layer of reality he can't perceive.
Now, he hides behind the tree.
He waits for us. Keeping himself alive, what little he can, for as long as he can.
Not too important. Not too unimportant.