Sonic the Hedgehog protagonist Sonic the Hedgehog, a blue hedgehog able to run at supersonic speeds, and recurring villain Dr. Eggman, an evil scientist whose ambition is to conquer the world. Among Sonic's many allies, the most well-known include: Miles "Tails" Prower, a twin-tailed, highly intelligent yellow fox who is Sonic's sidekick and best friend; Amy Rose, a pink hedgehog who is Sonic's self-proclaimed girlfriend and a hero in her own right; Knuckles the Echidna, a coolheaded red echidna who is the guardian of the Master Emerald; Shadow the Hedgehog, an anti-heroic doppelgänger of Sonic with a dark past; and Metal Sonic, a
Sonic the Hedgehog forms, or both. The methods used to acquire the Emeralds and the end-results after collecting them differ between games in the series. Most early games require the player to find the emeralds in Special Stages, while some modern games implement the emeralds as a plot element. In certain games, such as "Sonic R" and the 8-bit versions of "Sonic the Hedgehog" and "Sonic the Hedgehog 2", the player is required to find the Chaos Emeralds within the zones themselves. The Master Emerald was introduced in "Sonic & Knuckles" as a plot element. It resides in a shrine on Angel Island