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 both Super Smash Flash and Super Smash Flash 2, 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 over the black silhouette of the mystery character that it is about to face. By winning the challenge and defeating the opponent, the character becomes playable. In it will be the same although the tasks the player must complete to unlock the characters are still unknown.
In Super Smash Flash
|Blue||Clear Adventure Mode with Blade on at least normal difficulty.|
|Cloud||Complete 100-Man Melee.|
|Crono||Clear 3-Minute Melee.|
|InuYasha||Clear Adventure Mode without losing any lives.|
|Jigglypuff||Clear Adventure Mode on any difficulty with any character.|
|Lloyd||Clear Adventure Mode with any character after having one hour of total play time on Adventure Mode.|
|Luigi||Clear Adventure Mode with Mario on at least normal difficulty.|
|Mewtwo||Clear Adventure Mode with Pikachu and Jigglypuff on at least normal difficulty.|
|Mr. Game & Watch||Clear Target Test with every character but him.|
|Mr. Incredible||Clear Classic Mode with the original 13 characters.|
|Naruto||Clear Adventure Mode with the original 13 characters.|
|Shadow||Clear Adventure Mode with Sonic, Tails and Knuckles.|
|Super Sonic||Clear Classic Mode with Sonic on any difficulty without getting Game Over.|
|Young Link||Clear Classic Mode with Link on any difficulty.|
|Zero||Clear Adventure Mode with Mega Man X on at least normal difficulty without getting Game Over.|
In Super Smash Flash 2
Only one character has confirmed unlockable status in the full version of SSF2.
|Sandbag||Complete Classic Mode with using a random character on Normal difficulty.|
These three characters were unlockable in early demos for the sake of their initial inclusion in SSF2.
|Black Mage||v0.7||Play 10 Versus matches.|
|Captain Falcon||v0.8||Win a Versus match with every other character but him.|
|Jigglypuff||v0.9a||Clear all events.|
- In the "Challenger Approaching!" screen for SSF2, the silhouette of the McLeodGaming's fist logo can be seen.