Shadow the Hedgehog is the Ultimate Lifeform like Ethan Tidwell and member of Ultimate Defender. He and Ethan once live with Maria and Gerald Robotnik until a G.U.N Trooper came and destroy them leaving him and Ethan safe from their destruction.
In "Lake Guardians" He, Karai and Ethan investaged the strange activites in Lake Valor until they came to face with Team Galactic, the three fought them until the three lake guardains; Mesprit, Azelf, and Uxie came to help them only to be captured along with him, Karai and Ethan. That was when Batman, Lightning, Sailor Jupiter, Vegeta, Wonder Woman, Aqua and Serah came to the recuse.
Shadow can move at speeds rivaling Sonic's own and can perform many of his moves and tricks like the Spin Attack. His own unique abilities include the ability to use just one Chaos Emerald to induce Chaos Control and being able to create and throw Chaos Spears. His destructive power is also severely increased when he takes off his limiters as seen in one episode where he takes them off, subsequently single-handedly destroying an entire Metarex fleet. Though this leaves him drained of power if he takes them off for too long. His body was also able to take incredible punishment such as being slammed into solid surfaces and even being struck by powerful laser cannons. Shadow has also displayed fearsome combat skills, as in episode 73 he was able to get past individual challenges of an injured Sonic, Knuckles (with his rings off), and Tails (escaping traps with Chaos Control). He crashed through walls without dashing into them. He was also shown fighting his way through a large fleet of metarex soldiers, destroying them all.