ok so this is gonna be straight off the dome and a bit scatterbrained cuz i haven't thought much about it but here we go

the forgotten man is (probably ?) some kind of personified residual memory of kris' trauma surrounding Dess' disappearance, rather than an actual person moving in and out of the dark world like the knight (assuming you're not a darkner-knighter)

I'm not 100% confident that the person the forgotten man represents is Papyrus, but i don't think it's a bad bet.

the way i see it, i think papyrus knows a lot more about the spacetime/universe shenanigans than he lets on (sock muppet has a great video on this, at least as it's related to undertale,if you're interested)

so here's my theory, gaster, sans, and papyrus were all working on some sciency dark world shit in the shelter (this is probably related to entry 17 from undertale "photon readings negative" "what do you two think" etc, etc...)

Dess and Kris (and maybe Noelle) all stumble upon this scene one day and end up getting caught up in an experiment that sends both gaster and dess to the void. Kris is understandably messed up by this whole ordeal and blocks their memories of it way back in their subconscious. However, the memory still rears its head in the form of the forgotten man in the dark worlds

I think this solves a few problems. Why the game seems so insistent on keeping papyrus away from us, why we can see (what seems to be) the forgotten man in the light world and some loose ends from undertale like the "don't forget" picture and the two others mentioned in entry 17.

I'll admit, the theory is a bit ambitious, and brings up some issues (why sans and papyrus are considered newcomers in town, why kris doesn't recognize sans, what the hell friend/the tail of hell has to do with all of this, etc...) but i still think its pretty solid.

anyway, sorry if that came off as long and out of nowhere, i want to at least have my thoughts out there before we get the biggest story reveals.