Unlockable character

An unlockable character is a character in the  series 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 unlockable character.

By completing the in-game feat and defeating the opponent in a 1-stock match, the character becomes playable. In the original Super Smash Flash, this match would always take place on, but in Super Smash Flash 2, each character is fought on a different stage, being their home stage.

In Super Smash Flash 2
✅ It has been confirmed that all characters added after the initial release of Beta 1.0 will be unlockable in the full version of SSF2. However, it should be noted that from Beta 1.2.0 onwards, all unlockable characters have been available without needing to be unlocked, in the same vein as starter characters. This is only temporary, as they will go back to being unlockable in a future update.

In demo versions
These three characters were unlockable in early demos for the sake of their initial inclusion in SSF2.

Trivia

 * In the original "Warning! Challenger Approaching!" screen for SSF2, the silhouette of the McLeodGaming's fist logo could be seen.
 * The Challenger Approaching Alarm since demo v0.9a was ripped from Super Smash Bros. Brawl onwards.