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.
I
think 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.