The Best Hidden / Unlockable Characters !
Fighters
Reptile - Mortal Kombat
Reptile was not a playable character in the first Mortal Kombat. He was fightable however. Having both the skill sets of his fellow ninja's in the game.
Honorable Mention
Mega Man - Street Fight X Tekken
This is not the Mega Man you are looking for. Instead we got the Mega Man 1 box art version. Fans begged for Mega Man to be in a fighting game and this is what Capcom delivered.
Role Play Games
Magus - Chrono Trigger
Making it to Magus and beating him feels like it could be the end of the game. Of course it was only about halfway through the game. Late in the game you have the choice to let him join your team or not.
Honorable Mention
Cloud Strife - Final Fantasy Tactics
Everyone knows who Cloud Strife is from Final Fantasy 7 fame. Not everyone knows he was in a different game as well outside of FF7 spinoffs.Platformers
Luigi - Super Mario Galaxy
This was a Mario only game and you can tell because it doesn't have "Bros." in the title. After completing the game andcollecting 120 stars(which is not a simple task) Luigi becomes a playable character.
Honorable Mention
Knuckles - Sonic The Hedgehog 2
(Sega Genesis)
This one is not a simple matter of unlocking a character. You had to purchase a separate cartridge. This allowed you to play as the new character Knuckles in Sonic the Hedgehog 2.