Blue

Blue is a  fan character owned by Kingdom Castle Animations, created specifically to appear as a playable character in Super Smash Flash.

Character description
Similarly to the case of Blade, Blue also originates from the Sonic fan characters created by Newgrounds user, Equinox-Twilight, particularly deriving from the other fan character Flare the Hedgehog (said to be the "brother" of Sonic the Hedgehog) who mainly appeared in the KA Omake Flash animated series. User Topcat was also in charge of redesigning Blue to be completely unique specifically for the game, so he could be told apart from Flare; the name "Blue" may have been also be thought up by Topcat himself, but this is not confirmed. Something that has never been clarified is whether or not Blue was intended to be in the game that eventually became Super Smash Flash, which was not initially projected to be a  fan game but as a platformer action-oriented game starring Blade. This also makes Blue's relationship with Blade a mystery, though they have since been depicted as alter egos and friendly rivals in future works.

Blue's ultimate design came after Super Smash Flash was finally completed and released to public: he is a blue hedgehog with loose but long quills who wears red gloves and shoes with black accents and who wields a characteristic sword. Much like characters in the Sonic canon, he is able to use the Spin Dash and the Spin Jump, and he can also create and manipulate fire.

In Super Smash Flash
Blue debuts as a playable character in the original Super Smash Flash as a clone character of. His attacks are slightly more powerful than Blade's at the cost of speed. The fact he was made specifically for the game could mean Blue was projected to accompany Blade, but no evidence exists to confirm this, so why he stayed after Super Smash Flash became a Super Smash Bros. fan game is unknown.

In Super Smash Flash 2
Blue was previously confirmed to appear again in Super Smash Flash 2, coming with a new redesign but with some similarities to his Super Smash Flash counterpart. He would have more pronounced fiery attacks. However, in May 18, 2012, it was announced that he would be scrapped from the roster, with Blade and other fan characters joining him.

April Fools' Day 2019 joke
For April Fools' Day 2019, McLeodGaming announced a supposed remake for the original Super Smash Flash titled Super Smash Flash 1 HD which would have featured updated higher-quality sprites resembling those from Super Smash Flash 2; though Blue was not part of the original joke (as it included a prefabricated character selection screen with starter characters only), developer Tommy FriendAlias Le nonetheless created a mockup sprite for him (resembling his planned redesign intended for SSF2) as he thought "[the joke] wouldn't be fun without the outrageous characters" (referring to Blue and Mr. Incredible).

Trivia

 * Blue's sword seems to be a flamberg-type rapier. Note the hand shield under his right hand. This "Flamberg" somewhat emits fire, much like Blade's own katana, which emits electricity.