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 "Sonic the Hedgehog" project began with just Naka and Ohshima, but grew to involve two programmers, two sound engineers, and three designers. Yasuhara joined to supervise Naka and Ohshima and develop levels, and became the lead designer. He satisfied Naka's request for a simple, one-button design by having Sonic do damage by jumping. "Sonic the Hedgehog" was released in 1991 and proved a major success, contributing to millions of sales of the Genesis. The development team took the name Sonic Team for the game's release. Naka has referred to Sonic Team as only a "team name" at this point.