>>713274864
He is
1. the rarest Fun event
2. the Fun event at Fun value 66, Gaster being heavily associated with repeating sixes
3. the most unique and enigmatic Fun event, not a recolor or part of a set
4. a clearly defined individual with a face, unlike any other "candidate"
In truth, Mystery Man might not be Gaster, but it just means who the hell should Mystery Man be?