Hidden characters can range from special boss battles to actual playable characters. Depending on the game, hidden characters are often unlocked by meeting special conditions or entering codes. Hidden characters often become famous (or infamous) thereby becoming regular characters or running gags in a series. Sometimes developers will insert themselves in as Hidden characters as well as other well known figures from pop culture. Some famous examples of hidden characters are Roo, Master Hand, Akuma, Bill Clinton, and Reptile.
A hidden character can be a Cameo Appearance or part of the main roster. They are applicable to this concept if they are not immediately playable from the start of the game.