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
Any info for unlockable characters is unknown at the moment.

Trivia

 * When Black Mage was revelaed to be playable in demo v0.7, he was surprisely added as an unlockable characer by unlocked by playing 10 Brawls. For his following appearances, he was now, definitely, an starter character.