Mega Man X

Mega Man X, known in Japan as Rockman X (ロックマンＸ Rokkuman Ekkusu) or simply known as X for short, is the main character of the Mega Man X sub-series and an alternate incarnation of the classic Mega Man. He appears in Super Smash Flash as a playable character. Though initially meant to "return" in some way, he was replaced with the original Mega Man in Super Smash Flash 2.

Character description
Mega Man X was created by Dr. Thomas Light as the first robot to have the capability to think, feel and act independently. Fearing that X might become a threat to the world (or that the world wasn't prepared for him in the Maverick Hunter X version), Dr. Light sealed X in a capsule to test his circuits. Many years later, Dr. Cain discovered X and was shocked at his design as they were far more complex than anything yet ever seen. Dr. Cain used X's design specifications and mass-produced his own robots which would later be called Reploids. However, because of a design flaw, Reploids grew more violent and these violent Reploids were deemed Mavericks. Thus, the human government formed an organization designed to combat these violent Reploids which would be known as the Maverick Hunters. X then joined the Maverick Hunters, becoming part of Sigma's 17th Elite Unit. Due to his hesitation on the battle-field, X remained a B-Class hunter.

In Super Smash Flash
Mega Man X appears as the only starter character of the Mega Man universe (whose name was misspelled as "Megaman", without the X following). All of his attacks involved shooting different beams in a similar way to Samus. He also has voice sample making him one of the few third-party character to have them.

In Super Smash Flash 2
Beta Mega Man X (again misrepresented simply as "Megaman") was originally intended to appear in Super Smash Flash 2 using his Biometal from from Mega Man ZX, but he was completely replaced with Classic Mega Man after a unanimous vote among devs. The only thing known about his appearance was his Final Smash, the Copy X Transformation.

Trivia

 * Though he is not playable in the Super Smash Bros. series, he is one of five Super Smash Flash characters to later have a costume for Mii Fighters based on him in Super Smash Bros. for Nintendo 3DS and Wii U. He shares this distinction with Zero, Lloyd, Tails and Knuckles.