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 he's 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.
|Character||How to Unlock|
|Blue||Clear Adventure Mode with Blade on at least Normal difficulty.|
|Cloud||Complete 100-Man Melee with any character.|
|Crono||Clear 3-Minute Melee with any character.|
|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 on at least Normal difficulty without getting Game Over.|
Any information is unknown.
- When Black Mage was revealed to be playable in demo v0.7, he was surprisingly added as an unlockable character who could be unlocked by playing ten Brawls.
- Similarly, Captain Falcon was unlockable in demo v0.8 by winning a single Brawl match with every other character.
- In demo v0.9, Jigglypuff who is unlocked by completing all the events in the demo.