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.