Shadow the Hedgehog

=Backstory= Shadow the Hedgehog 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.

Memory
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.

Present
Shadow helped Sonic save the world countless times, in Sonic Adventure 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 "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.

=SSF2= Shadow is in SSF2 as a Playable Character.