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
"Sonic the Hedgehog 2" sold over six million copies, making it the second-bestselling Genesis game behind the original "Sonic the Hedgehog". It received highly positive reviews from critics, who commended the game's level design and visuals, although its multiplayer mode was criticized. It has been rereleased on various platforms; a remastered version developed using the Retro Engine released on iOS and Android in December 2013. Two direct sequels, "Sonic the Hedgehog 3" and "Sonic & Knuckles", were released in 1994.