>>529858421
it's gaster. he was a prestigious "big shot" who got framed for something kris did and had his life/career obliterated. he "took the fall"

asgore knowingly framed gaster, but also believes that kris is innocent and is trying desperately to find the "real" culprit.

the sad humor of the situation is that gaster himself has already moved on from it and is actually happy to see kris in chapters 2 and 4.