I don't think enough people talk about YOU TWO. People are always attributing various things to gaster, but there were two other people in the room with him when it happened. They could be the ones doing these things. I always felt like a lot of the things attributed to him seem to contradict each other personality-wise. Maybe one of them is going around giving random short guys new, never-before-seen complexes while the other is handing out eggs, or making fountains.

On deltarune.com/chair there's dialogue that appears in the tab name: "But what if it could get... ...darker than dark?" This seems like it was said surrounding one of Gaster's dark experiments, but notably, it is not in all capital letters, like Gaster always speaks in. This was someone else talking. They use capitalization the normal way, just like the voice that discards the vessel, so they could be the same person. I know just about everyone uses capitalization the normal way, the connecting thread here is both being in relation to Gaster, the capitalization just means that them being the same person is allowed.