The series features three fictional species, in order of appearance: Chao, which have usually functioned as digital pets and minor gameplay and plot elements Wisps, which have been used as power-ups and Koco, which when collected grant new abilities for Sonic, among other things.
These have ranged from anthropomorphic animal characters like Shadow the Hedgehog and Cream the Rabbit to robots created by Eggman like Metal Sonic and E-123 Omega, as well as human characters like Eggman's grandfather Gerald Robotnik.
The series has introduced dozens of additional recurring characters over the years. All five of these have remained major characters and appeared in dozens of games. Sonic 3 introduced Knuckles the Echidna, Sonic's rival and later, friend. Sonic CD introduced Amy Rose, a female hedgehog with a persistent crush on Sonic. The sequel, Sonic 2, gave Sonic a fox friend named Tails. 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). Clockwise from top: Doctor Eggman, Rouge the Bat, Shadow the Hedgehog, Knuckles the Echidna, Sonic the Hedgehog, Amy Rose, Miles 'Tails' Prower, Cream the Rabbit, Cheese the Chao (flying, center), and Blaze the Cat.
