Zero

Zero is a highly advanced and powerful robot created by Dr. Wily and one of the main characters in the Mega Man X sub-series. He is a playable unlockable character in Super Smash Flash.

Character Description
Zero was created by the late Dr. Wily sometime in the early twenty-first century. Schematic blueprints of his body were seen during Bass's ending in Mega Man 2: The Power Fighters. Learning from his past mistakes, including the accidental creation of Bassnium and his construction of Bass and King, Wily constructed Zero as a new robot, far more advanced than anything he had ever built before, with a power level far superior to that of Bass or Mega Man. Wily took his time to complete Zero, who he called his "greatest masterpiece". Zero was routinely activated in a semi-conscious state during his construction, and Wily spoke with him, mirroring what Dr. Light himself did with X. Wily originally planned to use Zero against both Mega Man and the insolent, rebellious Bass. However, this never happened, and Zero was never used during the Wily Wars. Zero was aggressive and disobedient, and because of this Wily decided to seal him in a capsule. Decades later, in the early twenty-second century, he was awakened by a group of Reploids prior to the first Mega Man X game.

When he was first activated, Zero awoke as a Maverick (by the definition of wanting to cause harm to all humans), and he destroyed all intruders who had dared to enter Wily's lair. Zero was later cornered by Maverick Hunter Elite, Gamma's Maverick Hunter unit inside an unknown facility—at the cost of the total loss of said unit, however. Not wanting to get anymore comrades endangered by the powerful enigmatic Maverick, then-Hunter Commander Sigma himself challenged Zero in an one-on-one battle. Even though Zero eventually gained the upper hand during the battle, Sigma was able to defeat the red Maverick after the gem on Zero's head began to glow and a 'W' symbol appeared, apparently causing Zero great pain. Sigma punched this crystal, shattering it and defeating Zero and, subsequently, passing on the Maverick Virus to himself. Zero was then brought to Dr. Cain, who analyzed him. After that, he recovered from his battle with Sigma and was placed under Sigma's watch. Zero did not show any signs of Maverick behavior and was enlisted into the Maverick Hunters' seventeenth Elite Unit.

In Super Smash Flash
Zero appears in Super Smash Flash as the only unlockable character of the Mega Man universe. He will challenge the player only by clearing Adventure mode with on normal difficulty or higher without continuing, where defeating him would unlock him. His sprites are ripped straight from Mega Man X3.

In Super Smash Flash 2
Zero was previously confirmed to return in Super Smash Flash 2 as part of the initial roster for the game, with a new design based on his appearance in Mega Man Zero. However, following the release of v0.7 of the SSF2 Demo, the developers revamped the whole roster in order to accommodate some new, unexpected characters. Currently, Zero's fate as a playable character is unknown.

Trivia

 * Though he is not playable in the  series, he is one of seven Super Smash Flash characters to later have a costume for Mii Fighters based on him in and Super Smash Bros. Ultimate. He shares this distinction with Mega Man X, Isaac, Chibi-Robo, Lloyd Irving, Miles "Tails" Prower, and Knuckles the Echidna.
 * Zero also appears in Super Smash Bros. Ultimate as an Assist Trophy, making him one of four characters in the SSF series to later appear as Assist Trophies in the official games. He shares this distinction with Bomberman, Knuckles, and Shadow the Hedgehog.
 * Zero's English voice actor in Marvel vs. Capcom 3: Fate of Two Worlds and Marvel vs. Capcom: Infinite, Johnny Yong Bosch, is also the English voice actor of Ichigo Kurosaki from the  franchise.