Sonic the Hedgehog
The Sega Genesis had "add-on" systems that incorporated "Sonic" games. "Sonic CD", released for the Sega CD, was a 2D platform game released in 1993. The game introduced the characters Amy Rose and Metal Sonic and featured levels that differed depending upon whether Sonic was in the past, present, or future time frames. "Knuckles' Chaotix", a spin-off released in 1995 for the Sega 32X, featured Knuckles and a new group named Chaotix fighting against Dr. Robotnik. The game featured a two-player cooperative system in which the on-screen characters were connected by magic rings. There was also "Sonic Eraser", a puzzle game released exclusively on Sega Game Toshokan in 1991.
Sonic CD
The 1991 release of "Sonic the Hedgehog", the first game in the "Sonic" series, was a major commercial success and positioned Sega as Nintendo's main rival in the console market. Lead programmer Yuji Naka, dissatisfied with Sega of Japan's rigid corporate policies, moved with several members of Sonic Team to the United States to develop "Sonic the Hedgehog 2" with Sega Technical Institute (STI). Meanwhile, Sega planned to release the Sega CD add-on for its Genesis, and wanted a "Sonic" sequel that would demonstrate its more advanced features. Sonic's character designer Naoto Ohshima was "Sonic CD"s director; the remainder of the team comprised Sega staff who had developed "The Revenge of Shinobi", "Golden Axe 2", and "Streets of Rage".