Mirror Finns appear in the episode "Gotcha!" after Lumpy Space Princess goes through a portal and ends up in a room full of cursed mirrors. According to Princess Bubblegum, evil things happen when one looks into the mirrors. Lumpy Space Princess sees Finn take his shirt and hat off and then walk towards her. As he reaches out towards her, his hand becomes black. The Mirror Finns then attack Lumpy Space Princess until Jake and the real Finn come in and rescue her. With each one Finn and Jake defeat, the mirror it came out of shatters.
While in the mirror, Mirror Finns look exactly like Finn, but when they step out, they appear as three-dimensional shadows of Finn. They have straight, waist-long hair. In her book, Lumpy Space Princess describes them as even more attractive than Finn.
The Mirror Finns may be a reference to The Legend of Zelda games where black or dark gray versions of the game's protagonist, Link, are recurring enemies. In one game it is revealed that these "Shadow Links" are created by the Dark Mirror.