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 other uses in various games, such as entering Special Stages in "Sonic the Hedgehog 2", and leveling up in "Sonic Heroes". In the 2D games, checkpoints take the appearance of posts, while in 3D games they are either small gates or pads on the ground. Springs are a staple in the "Sonic" series. They are scattered throughout the levels and serve to catapult the player at high speeds in a particular direction. Sometimes they allow the player to proceed further in the level, while other times they are used to hinder the player, usually by sending Sonic towards a dangerous