Alternate-Universe Squidward is a creature who looks exactly like Squidward Tentacles, except he is confined within mirrors or appears in an alternate universe.
It is unknown what universe he is in or if he is an illusion. Although it is likely that he is in the same reality as Alternate-Universe SpongeBob and Alternate-Universe Patrick.
Like Alternate-Universe SpongeBob, AU Squidward seems to like his counterpart as they attended a tour together and got along.
Alternate-Universe Squidward appears several times during the episode. He appears whenever Squidward looks into something reflective (his glass, a mirror). AU Squidward was the person Squidward took to the Galleria Diphtheria as Squidward made SpongeBob carry a mirror. In the end, AU Squidward told the real Squidward he'll always be his plus one which delights the latter.