Mirror Finn, also known as Nnif, appears 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. Mirror Finn and his duplicates then attack Lumpy Space Princess until Jake and the real Finn come in and rescue her. With each of Mirror Finn and his clones defeated by Finn and Jake, the mirrors came into shatters.
While in the mirror, Mirror Finn looks exactly like Finn, but when he steps out, he appear as three-dimensional shadows of Finn. He has straight, waist-long hair. In her book, Lumpy Space Princess describes him as an even more attractive person than Finn.
- Mirror Finn may be a reference to The Legend of Zelda games where the black or dark gray version of the game's protagonist: Link is a recurring enemy. In one game, it is revealed that "Shadow Link", also known as "Knil", is created from the Dark Mirror.