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.
List of Sonic the Hedgehog characters
The "Sonic the Hedgehog" video game franchise began in 1991 with the game "Sonic the Hedgehog" for the Sega Genesis, which pitted a blue anthropomorphic hedgehog named Sonic against a rotund male human villain named Doctor Eggman (or Doctor Ivo Robotnik). The sequel, "Sonic 2", gave Sonic a fox friend named Tails. Shortly afterward, "Sonic CD" introduced Amy Rose, a female hedgehog with a persistent crush on Sonic, and "Sonic 3" introduced Knuckles the Echidna, Sonic's rival and, later, friend. All five of these have remained major characters and appeared in dozens of games.