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 2 was released in 1994, two years after "Sonic 2". The "Sonic & Knuckles" game cartridge features a special "lock-on" port into which the player can insert other Genesis cartridges. Attaching "Sonic 2" unlocks "Knuckles the Echidna in Sonic the Hedgehog 2", a variation of "Sonic 2" whereby the player plays as Knuckles the Echidna, a character introduced in "Sonic the Hedgehog 3" and not playable until "Sonic & Knuckles". Though the game is largely identical to "Sonic the Hedgehog 2", Knuckles is able to glide and climb walls, allowing him access to areas previously inaccessible to Sonic or Tails. However,