Shadow the Hedgehog

Shadow also known as "The Ultimate Lifeform" first appeared in Sonic Adventure 2 for the Dreamcast console. Shadow was created for the "Ultimate Good" but government thought Professor Gerald, Shadow's creator might use him for evil, so they sent G.U.N. soldiers to shut down the Space Colony ARK. Shadow is the rival to Sonic.

Past
Shadow's memories were said to be erased, but he tends to have random flashbacks. Geralds's granddaughter Maria (who is most likely either Dr. Robotnik (Eggman)'s sister or cousin seeing as they both share the same grandfather; but Sega has not confirmed this.) sent Shadow to earth before being shot by a G.U.N. soldier. Shadow promised to get revenge on humanity for the sin. Also, his partner/best friend was Maria Robotnik (Gerald's granddaughter). But then, G.U.N. then decided to shut down Gerald's space station, kiling Maria. But, before she died, Maria launched Shadow into a launch pod and sent him down to Earth. And, since then, Shadow destined to destroy Maria, although she still remains in his heart.

Present
Shadow helped Sonic save the world countless times, in Sonic Battle (GBA) it is revealed that Shadow can't be the enemy of humanity because he has the heart and soul of Maria. After killing his techinal brother bio-lizard and "technical father" "Black Doom" (whom Shadow was created from via his blood), Shadow decided to put his past behind him after finding out his purpose and decided do do what he wanted to do with his life. He became a G.U.N. agent.

Brawl
Shadow made an appearance in SSBB as an assist trophy, he uses Chaos Control, his signature move, to slow down all except the team that summoned him.

Super Smash Flash
Shadow made an appearance in Super Smash Flash as one of the 28 characters. He was unlocked only after Sonic, Knuckles and Tails all cleared Adventure Mode.

Super Smash Flash 2
Shadow once again appears in SSF2 as an unlockable character. He will be faster and stronger than in SSF.

Other
Entrance: A black hole appears and Shadow comes out of it

Taunt: Takes a serious pose

Wins: Takes out a Chaos Emerald

Loses: Touches his head like he is in pain