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 Team
The Saturn did not achieve the commercial success of the Genesis, so Sega focused its efforts on a new console, the Dreamcast, which debuted in Japan in 1998. The Dreamcast was seen as opportunity for Sonic Team to revisit the "Sonic" series, which had stalled in recent years. Sonic Team was originally creating a fully 3D "Sonic" game for the Saturn, but development moved to the Dreamcast to align with Sega's plans. Takashi Iizuka led the project; Iizuka had long wanted to create a "Sonic" role-playing video game and felt the Dreamcast was powerful enough to achieve his vision. The game became "Sonic Adventure", launched in 1998, which became the bestselling Dreamcast game.