In the first game Sonic had to liberate animals and defeat Dr. Robotnik. He returned in every level. The sequals were more or less excact copies, altough better implemented. But there were other games, like the pinball game Sonic Spinball.
Just like with Mario there was a lot of merchandising and a cartoonserie. One year Sonic was the name on every Schoolyard. It wasn't to be for ever though. Sega lost its market share. The dreamcast still had a Sonic adventure, but know he has traveled to the other side: there is a Sonic for the Gameboy Advance.
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