Because Nintendo had Mario, Sega wanted to have a symbol as
well. He was found in Sonic. He replaced Alex Kidd. Naoto
Oshima designed the character and Yuji Naka programmed the
first game. Sonic was everything Mario wasn't. Instead of
being a plumber, he had attitude. Mario was slow, speed was
the central element in Sonic games. No strange little hat,
but cool sneakers.
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.