Shadow the Hedgehog

Shadow the Hedgehog, also known as "The Ultimate Lifeform", is a character from the Sonic franchise. He first appeared in Sonic Adventure 2 for the Dreamcast console. Shadow was created by Professor Gerald Robotnik for the purpose of providing genes to cure the deathly ill, such as his granddaughter Maria. Shadow stayed at the Space Colony ARK, forming a close bond with Maria. Soon however, when the government had growing concerns over Gerald's work on immortality, the G.U.N invaded the ARK, killed Maria and locked away Shadow for fifty years before he was freed by Dr. Eggman.

Character description
During the development of Sonic Adventure 2, it was decided that the motif of the game was to be between good and evil. To do this, the development team would create new characters designed to assist Dr. Eggman to combat the main protagonists of the Sonic series. Yuji Uekawa came up with a darker version of Sonic who was then dubbed "Terios" which means reflection of. During much of the development of Sonic Adventure 2, the Sonic Team did its best to keep Shadow's identity a secret in order to build up the hype for the game.

In Super Smash Flash
Shadow made an appearance in Super Smash Flash as an unlockable character. He was unlocked only after ,, and  have cleared Adventure mode.

In Super Smash Flash 2
Shadow was previously confirmed to return in Super Smash Flash 2 as part of the initial roster for the game. 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, Shadow's fate as a playable character is unknown.

Trivia

 * Though he is not playable in the Super Smash Bros. series, he is one of seven characters in the Super Smash Flash series to appear as Assist Trophies in the official games. He shares this distinction with Waluigi, Isaac, Krystal, Knuckles, Zero, and Bomberman.