Unlockable character

An unlockable character is a character that is not available at the start of the game (unlike a starter character), but must be unlocked. Unique criteria must be met in order to unlock a certain character. In Super Smash Flash, typically, the player must accomplish a particular in-game feat and once complete, a message will display: "Warning! Challenger Approaching!" with a white question mark block of the mystery character that he's about to face. By winning the challenge and defeating the opponent, the character becomes playable. All unlockable matches takes place in Final Destination. In Super Smash Flash 2 it will be the same although the tasks the player must complete to unlock the characters are still unknown.

Super Smash Flash 2
Since this game is still under development, no characters have been confirmed for to be unlockable, but some are beleive to be one like the prequel.