Eclypsed
Eclypsed in his universe was a sort of guardian who had to protect his land even though he had no role model, so he met Caelus who eventually took him under his wing.
Unfortunately, one day a catastrophe occurred in his universe and since Eclypsed was not properly prepared, Caelus used all his magical strength and opened a portal into which he threw Eclypsed, thus sacrificing himself.
Eclypsed was transported to a white void where he had to start fending for himself until, a few years later, he met Swap Sans and his Papyrus who welcomed him.
Since he could no longer continue his work as a guardian, he decided to visit various AUs both out of curiosity and for a sort of obsessive control; these movements of his are endless and this is why he has dark circles under his eyes.
He still thinks that Caelus and his universe are still somewhere and alive.