From Shadowpedia
According to official profiles, Shadow was created long ago by Prof. Gerald Robotnik to be the Ultimate Life Form. Sharp witted and seemingly always on the edge, Shadow is described as the dark incarnation of Sonic the Hedgehog. Once he has set himself to a goal, he will do whatever it takes to accomplish it, regardless of any danger. After the trauma of the death of his first closest friend, Maria Robotnik, Shadow strives to fulfill his purpose and keep the promise he made to her, despite his general loathing of mankind. Although his relationship with Sonic seems to have developed from antagonistic to friendly rivalry, animosity from not understanding their different mindsets continues.
Shadow is ageless and immortal. However, Shadow shares a lot in common with Sonic. Although Sonic's speed is described as "second to none," Shadow can skate at speeds fast enough to rival the speeds the world's fastest hedgehog can run. He can also perform spin attacks common to Sonic, which are a variations on the tendency for hedgehogs to roll into tight balls for protection. Additionally, with the power of a Chaos Emerald, Shadow can warp time and space with Chaos Control. In contrast, the Archie Comics version of Shadow can directly access the "Chaos Force" and does not require a Chaos Emerald. Shadow is also able to use a variety of other Chaos powers, such as "Chaos Spear" and "Chaos Blast".
By harnessing the power of the seven Chaos Emeralds, Shadow is able to use super transformation to transform into Super Shadow. He is bestowed with the new abilities of flight and near invulnerability, like Super Sonic, and his normal abilities of speed and his Chaos powers are greatly enhanced.
