>>713833806
>What similarities?
The letter is described as "strange"
The writing is "illegible"
There are frequent and unusual line breaks
It directly acknowledges "DELTA RUNE"
When it's finished, it suddenly and inexplicably disappears

The Man is always described vaguely and indirectly, "Well, there is a man here," "He is behind the tree." Similarly, Gaster's theme is simply "him," and the sprite universally assumed to represent him is "mysteryman."
The Man has been forgotten by everyone and can only exist in places that shouldn't exist, rooms between rooms that can't be found and disappear when you leave them. It's also normally impossible for him to communicate with you within the game, even if you do manage to find him. Both of these qualities also seem to apply to Gaster.