Wait a minute…
Dess, Papyrus, and Ralsei are clopen beings in Deltarune. There, but not there. When Ralsei is damaged too badly, he disappears. When Papyrus is behind an object, after you interact with him, it’s as if he never existed at all. Dess is the same thing. There, but not really there.

Gaster is clopen in Undertale but only somewhat in Deltarune.

Also… there is a certain Undertale NPC who will be brought up soon.