In "Roller Cowards," AU-Patrick called Patrick a "big fat crybaby." Patrick Star responded by punching him and saying "I thought we settled this the last time!," implying that he and Patrick encountered each other before. In "Rule of Dumb," AU-Patrick looks like Patrick, but with a unibrow, spines on his hunchback, a nose, and large pointed bottom teeth. Patrick and AU-Patrick can only see each other using a mirror. They can reach into each other’s worlds (proven by Patrick, when he punches AU-Patrick). Patrick and AU-Patrick don't like each other very much. He also appeared in "The Great Patty Caper" when Patrick suggests that his reflection in the window looks suspicious. Afterwards, the reflection gains a smug expression, then walks away.
His most recent appearance was in an unnamed SpongeBob comic in the Nick MagazineSeptember2009 issue. In the comic, SpongeBob and Patrick receive a treasure map from an elderly sailor. SpongeBob reminds Patrick of the curse that the sailor told them off. When SpongeBob then tells him about what the sailor said, a bubble comes up with Alternate-Universe SpongeBob, Alternate-Universe Patrick and Alternate-Universe Elderly Sailor reenacting when the sailor giving the map to them. When Patrick denies that even happened, the Alternate-Universe Sailor slaps him and says "Sure I did! Pay attention, sonny!" Then, in the background, Alternate-Universe Patrick is laughing at him and Alternate-Universe SpongeBob is staring at SpongeBob with a blank expression.
"I am you and you are ME!!!"
"(Both Patrick and Mirror Monster) AH! WHAT HAVE I BECOME!?"
"You! I don't get it"
Unlike Alternate-Universe SpongeBob, Alternate-Universe Patrick is always shown to be evil as he tells Patrick that he is a crybaby and even laughs at him when he gets smacked by the Alternate-Universe Sailor.
The Mirror Monster might be based off an old theory on how the mirror world is actually filled with dark energy such as monsters.