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 collect one of the Chaos Emeralds. In "Sonic the Hedgehog 3", if all Emeralds have already been found, touching them rewards the player fifty rings. In most games since "Sonic Adventure 2", these giant rings serve as the end level marker, which ends the level upon touching it. Checkpoints are items placed throughout the stages in "Sonic" games which serve mainly as progress markers. If the player runs through one, their progress through a level is "saved". If the player then loses a life on the same stage, they will start over at the last checkpoint passed. Checkpoints also serve