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 Heroes
Sonic Heroes is a 2003 3D platform game in Sega's "Sonic the Hedgehog" series. The player races a team of series characters through levels to amass rings, defeat robots, and collect the seven Chaos Emeralds needed to defeat Doctor Eggman. Within each level, the player switches between the team's three characters, who each have unique abilities, to overcome obstacles. "Sonic Heroes" abandons the action-adventure and exploration-based gameplay of its predecessors "Sonic Adventure" (1998) and "Sonic Adventure 2" (2001) and instead returns to the linear style of Sega Genesis-era "Sonic" games.