Dr. Eggman is a fat person who is sonic's biggest rival in two ways. 

Dr. Eggman is know for his big mustache and his inssesive amount of robots. He wears a big red, white, and yellow coat, He wears white gloves and long black pants with black shoes, He also wears goggles, He is also known for his intelligence.

Dr. Eggman's first apperance was in Sonic The HedgeHog, His latest apperance is in Sonic Boom. His first name was Dr. Robotnik, Or like sonic likes to call him Dr. Robuttnik.

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Sonic Boom™ - Video Game Reveal Trailer

Sonic Boom™ - Video Game Reveal Trailer

This is just the trailer