The goal of Project Shadow was to create immortality and it succeeded with Shadow. He’s actually the result of a military-funded science experiment titled Project Shadow and, upon his creation, he was dubbed the ultimate life form. While it’d be easy to assume that he’s Sonic’s evil brother/clone, Shadow’s origins are far more complex.
For anyone less familiar with the character, let’s run through his backstory from the games (primarily Sonic Adventure 2). Given his popularity, it only makes sense for him to feature in the movies. Yes, Shadow the Hedgehog makes a last minute cameo.
Sonic The Hedgehog 2 post credits scene explained – who is Shadow the Hedgehog?