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 the Hedgehog 2
Due to the popularity of its predecessor, "Sonic the Hedgehog 2" already had an established fanbase anticipating its release. The game received critical acclaim and was a bestseller in the UK charts for 2 months. As of 2006, the game had sold over 6 million copies, making it the second best-selling game for the Sega Genesis (after the original "Sonic the Hedgehog").