Metal Sonic was created in the image of Sonic the Hedgehog and is thus mostly built of blue metal. He has a yellow circular depression in his chest with a black center, mirroring the peach skin on Sonic's chest. His head has metal fins resembling Sonic's quills and a pair of triangular "ears." Metal Sonic has a muzzle, shoulder plates, upper arms, legs, and fingers made out of silvery metal. His palms are black, while the backs of his hands are square yellow plates. His forearms and lower legs are blue metal like his head and torso, and he has red feet with white stripes designed to resemble Sonic's shoes. He has optic sensors with black sclera and red irises.

Following the Ifrit incident, Metal Sonic received a slight update in design which made him taller and gave him a more ellipsoid torso. He also gained more detailed hands, a brilliant blue paint job, joints on his arms and legs, and sharper eyeholes.

Despite being built to match Sonic the Hedgehog in all aspects, Metal Sonic is the exact opposite of his organic counterpart in terms of personality. Rather than fun-loving, adventurous, and carefree, Metal Sonic is a cold, dark, aggressive, mysterious and outwardly emotionless machine with a large superiority complex. Early on though, he had a habit of mimicking Sonic's gestures, perhaps in a mocking manner. Notably, Metal Sonic is also completely mute,[6] being only capable of making beeping and buzzing noises.

Most of the time, Metal Sonic is Dr. Eggman's docile and completely obedient enforcer. In addition to being programmed to serve Eggman, Metal Sonic carries only one goal in mind: destroy Sonic using abilities superior to his. However, he is far from being another mindless Badnik. In fact, Metal Sonic is an intelligent and calculative machine thanks to an AI chip given to him by Eggman. Also, in contrast to his emotionless demeanor, he is able to feel emotions and carries a deep capability for vengeance. These traits together has allowed Metal Sonic to fuel his own dark goals of destruction and domination. In battle, Metal Sonic is likewise machine-like in execution and coldly serious. Combined with his muteness, he is a silent killer in every sense of the definition.