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 Adventure
During the early 1990s, Sega was one of the most successful video game companies due to its Genesis console. Sales for the console were driven by the popularity of its flagship franchise of 2D platformers, "Sonic the Hedgehog". During this time, series co-creator Yuji Naka worked with Sega Technical Institute (STI) in the United States to develop "Sonic" games. After the completion of "Sonic & Knuckles" in 1994, Naka moved to work with Sonic Team in Japan. STI began developing "Sonic X-treme" for the Sega Saturn"," which would have been the first "Sonic the Hedgehog" game to feature full 3D gameplay. "X-treme" suffered a series of setbacks and was canceled in 1997. The cancellation is an important factor in the Saturn's commercial failure; without it, the system had no original "Sonic" platformer. Meanwhile, Naka and Sonic Team developed original Saturn games, such as "Nights into Dreams." Due to the series' relative lack of presence on the Saturn, according to "Retro Gamer", "[b]y mid-1997 Sonic had essentially been shuffled into the background... it was astonishing to see that just six years after his debut, Sonic was already retro."