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. Sonic CD introduced Amy Rose, a female hedgehog with a persistent crush on Sonic. 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.
The series has introduced dozens of additional recurring characters over the years. These have ranged from anthropomorphic animal characters such as Shadow the Hedgehog and Cream the Rabbit to robots created by Eggman such as Metal Sonic and E-123 Omega, as well as human characters such as Eggman's grandfather Gerald Robotnik. 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.
The Sonic games keep a separate continuity from the Sonic the Hedgehog comics published by Archie Comics and other Sonic media and, as a result, feature a distinct yet overlapping array of many characters.