Nearly all games in the series feature a blue hedgehog named Sonic as the central player character and protagonist. The games detail Sonic and his allies attempt to save the world from various threats, primarily the evil genius Dr. Ivo "Eggman" Robotnik, the main antagonist of the series. Robotnik's aim is to rule the Earth; to achieve this, he usually attempts to eliminate Sonic and to acquire the powerful Chaos Emeralds.
The first Sonic game, titled Sonic the Hedgehog, was a platform game released in 1991[1] that featured protagonist Sonic running through the game's levels in order to foil Doctor Robotnik's attempt to take over the world.
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http://en.wikipedia.org/wiki/Sonic_the_Hedgehog_(series)