9. Facing Reptile
While playing the original Mortal Kombat, a mysterious green ninja sometimes appeared before a fight and said something cryptic like, "Blocking will get you nowhere" or "Perfection is the key". Players eventually realised that this ninja, who is called Reptile, was a secret character you could fight. Not only that, he was giving you instructions on how to face him.
To face this Zaterran warrior, the player must perform a Double Flawless victory without blocking on The Pit level and perform a Fatality. This will only work if a silhouette of Santa Claus on his sleigh appears in front of the Moon. (That sounds made up but I promise it is true). If you achieved this criteria, Reptile will challenge you at the bottom of The Pit.
Facing Reptile wasn't just a big deal because he was a secret character. He was the first secret character EVER in a fighting game! Nowadays, almost every fighting game has at least one hidden character. Akuma in Super Street Fighter II. Devil Jin in Tekken 5. Mewtwo in Super Smash Bros. Melee.
As memorable as it was to discover these characters, Reptile is the one who started it all.