Board Thread:General Discussion/@comment-28009984-20160319184042/@comment-27940905-20160329004636

Gaster cannot have been any of the fallen humans.

He erased himself from reality and is implied to be omniscient or omnipresent, but only after he built the CORE. Gaster is an incredibly meta concept, in that he's dummied out content and is apparently self-aware about it, and understands that nothing changed with his disappearance, and all evidence of his existence is either missing or obscured. Since the other human souls (the non-red ones, anyway) are all accounted for, none of them can be his.