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 3 to it. Connecting the "Sonic 3" cartridge creates a combined game, "Sonic 3 & Knuckles". The lock-on function is available in some digital releases of the games, such as the Virtual Console service for the Wii. "Sonic the Hedgehog 3 & Knuckles" allows the player to play "Sonic 3" levels as Knuckles or "Sonic & Knuckles" levels as Tails or both Sonic and Tails. Other new features are the ability to collect Super Emeralds, unlocking new "Hyper" forms for Sonic and Knuckles and a "Super" form for Tails, improved save options, access to new areas that Sonic or Tails could