In the original Mortal Kombat (1992) for arcade, there aren't any hidden characters that can be "unlocked" and played, much like in the later iterations of the game. However, there exists a hidden opponent named Reptile, whom you can fight against, but can't play as. To fight against Reptile, you have to meet certain conditions: 1. Be in the pit stage. 2. Get a double flawless victory (win both rounds without getting hit) and finish your opponent with a fatality (it varies by character so you'll have to look up your chosen player's fatality). 3. See a silhouette fly past the moon. Doing all this, under these very specific conditions, you will be transported to the bottom of the pit where you will face off against Reptile on a mirrored background. But remember! This is no easy task and the conditions may take a few tries to meet due to the difficulty of achieving a double flawless victory. Good luck! Please also keep in mind that future versions of the game and newer platforms might have different rules or additional hidden characters, so it's always worth checking specific guides for the version/platform you're playing on. Happy gaming!