Many video games these days, especially fighting games
, include hidden characters
as an incentive to play the game.
By beating the game, beating a level a special way, doing a special button combination, whatever, you can add to the list of available characters.
the game that started this was the Mortal Kombat
series. Mortal Kombat 1
had a hidden enemy dropping down with hints occasionally. By Mortal Kombat 3
there was a fairly large list of hidden charaters tacked on.