They have the same basic similarities thanks to being the same species (not to also mention abilities), That's really it. There's a reason why people with barely any knowledge of Sonic will call Shadow either Black or Evil Sonic. That and in SA2, People mistake Shadow for Sonic more so due to progress of elimination then looks alone (A Male Hedgehog capable of blitzing superspeed and to make.
In Sonic 3, Sonic, Tails and Knuckles are called by G.U.N. to battle against Shadow the Hedgehog, who has escaped from his animated paralysis prison in Japan. Upon first meeting Shadow, the trio are shocked by Shadow's appearance, leading to Sonic questioning why they look alike.
Both Shadow and Sonic are similar in their physical appearances, but while Sonic is recognizable by his blue fur. Gerald Robotnik modeled him after the depiction of Super Sonic defeating Perfect Chaos found in the Ancient Ruins. Back in the Adventure Era, Super Sonic's quills didn't stand completely up like they do in the current design so shape wise he and Shadow looked quite similar.
Shadow's dark coloration could have something to do with having some of Black Doom's DNA. Why does Shadow look like Sonic? Shadow the Hedgehog's obvious resemblance to Sonic might lead some to believe that the character is an evil clone of Sega's fast-paced mascot. Shadow is the result of genetic experimentation, but he's got nothing to do with Sonic and their resemblance is merely coincidental.
Sonic boom is the most diverse the cast got in shape and size and there is still a purposefully design to have Shadow look a lot like sonic but in a different hairdo. Why Does Shadow Look Like Sonic? December 19, 2022 Shadow the Hedgehog is a fictional character in the Sonic the Hedgehog video game franchise. He is an anthropomorphic hedgehog with red quills, black fur, and a gold ring on his chest.
This would imply that the physical differences are mostly for the audience to differentiate them, and in canon Shadow and Sonic actually look a lot alike. Or that Hedgehogs are colorblind. Why are Sonic and Shadow so similar? Based on the story, there isn't really anything connecting them.
From what I can tell, they're just two characters who happen to both be hedgehogs who can run extremely fast, who happen to look very similar to one another, and who happen to have the same basic set of abilities. And that's stupid, in my opinion. Sonic Team plays the two characters as rivals.
Dr Gerald Robotnik saw a mural of Super Sonic fighting Perfect Chaos, which is why Shadow looks like Sonic, part of his quills are pointed up, and the Biolizard resembles Perfect Chaos.