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 completely playable under a second player's control, as well as adding the option for players to utilize Tails' flying ability in levels. It also introduced the character, Knuckles the Echidna, who served as an additional antagonist with Doctor Robotnik for the game. "Sonic & Knuckles", another platform game in the "Sonic series", was released later in 1994. The game featured Knuckles as a playable character with gliding and wall climbing abilities and allowed gamers to plug in "Sonic the Hedgehog 3" to the top of the "Sonic and Knuckles" cartridge as part of the game's "lock on" functionality. This allowed