Knuckles the Echidna in Sonic the Hedgehog 2

Knuckles the Echidna in Sonic the Hedgehog 2 (or simply Knuckles in Sonic 2) is a platform video game which is the result of locking-on Sonic & Knuckles with Sonic the Hedgehog 2 using the former catridge's unique "Lock-On" technology. As the name suggests, this game is essentially identical to Sonic the Hedgehog 2 with the exception of Knuckles the Echidna starring as the main playable character, and Sonic the Hedgehog and Miles "Tails" Prower are not playable in the game.

Gameplay
As Knuckles, he retains his moveset from Sonic & Knuckles, allowing him to glide in the air and Climb walls with ease, which allows him to access areas otherwise unreachable by Sonic and Tails, while his weaker jumping abilities make some situations, such as certain boss fights (particularly the final boss of Death Egg Zone), more difficult.

When Knuckles collects all 7 Chaos Emeralds from Special Stages, he can transform into Super Knuckles while holding 50 Rings.

Changes
There are also numerous changes made in Knuckles in Sonic 2 which include:
 * The 2P Player VS and Options menu have been removed from the title screen. As such, there is only simply a "Press Start" option resembling the title screen of the original Sonic the Hedgehog.
 * The color palettes for the title cards are colored green, red and yellow instead of blue, red and yellow.
 * The Shield power-up is colored gray instead of its usual blue.
 * The Signpost will spin to Knuckles' portrait instead of Sonic (or Tails). The said portrait appears to be reused from Sonic & Knuckles.
 * Secrets such as 1-Ups have been added in the Acts for Knuckles to discover.
 * When Knuckles respawns at a Star Post after dying or finishing a Special Stage, he will retain the number of Rings he had when he touched it. This makes it easier to enter other Special Stages from other Star Posts in that act.
 * In Special Stages, Knuckles' Ring requirements to pass each checkpoint are notably lower than Sonic and/or Tails' Ring requirements, making the Special Stages much easier for Knuckles. In conjunction with the previous point, this also makes achieving Super Knuckles much easier compared to achieving Super Sonic.
 * Activating Super Knuckles requires the player to press the jump button twice as opposed to a single jump. In addition, the player cannot transform into Super Knuckles after touching the Signpost or pressing the switch on the Capsule.
 * In Debug Mode, the player is limited to placing a Ring or a Super Ring Monitor.

Cheats

 * Act Select: At the title screen, press the following in order: Up, Up, Up, Down, Down, Down, Left, Right, Left and Right. (A Ring chime will sound if successful.) Hold and press "Start" to access the Act Select.
 * Debug Mode: At the Act Select Sound Test, play the following tunes in order: 01, 09, 09, 04, 01, 00, 01, 08. (A Ring chime will sound if successful.) Then highlight any Act and select it while holding to start an Act with Debug Mode.
 * All 7 Chaos Emeralds: At the Act Select Sound Test, play the following tunes in order: 01, 06, 07, 07, 07, 02, 01, 06.

Re-releases

 * Sonic Jam (1997)
 * Sonic Mega Collection (2002)
 * Sonic Mega Collection Plus (2004)
 * Sonic Mega Collection Plus & Super Monkey Ball Deluxe (2005)
 * Sega Fun Pack: Sonic Mega Collection Plus & Shadow the Hedgehog (2009)
 * ''Sonic PC Collection (2009)
 * Sonic Classic Collection (2010)
 * Virtual Console (2010)

Trivia

 * In Special Stages, Knuckles' name is not displayed like Sonic and Tails'.
 * The Debug Mode code (01, 09, 09, 04, 01, 00, 01, 08) references the release date of Sonic & Knuckles (18 October 1994).
 * This is also the reference used for the Debug Code in Sonic the Hedgehog 2.