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 cartridge'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.
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.
There are also 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 Act title cards are colored green, red and yellow instead of blue, red and yellow.
- The Shield is colored gray instead of its usual blue.
- The Signpost will spin to the portrait of Knuckles portrait instead of Sonic (or Tails). It appears to be reused from Sonic & Knuckles.
- Secrets such as 1-Up Monitors have been added in the Acts for Knuckles to discover.
- When Knuckles respawns at a Star Post after losing a life 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 during a jump (as opposed to simply jumping into the air). 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.
- 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.
- Sonic Jam (1997)
- Sonic Mega Collection (2002)
- Sonic Mega Collection Plus (2004)
- Sonic Mega Collection Plus & Super Monkey Ball Deluxe (2005)
- Xbox Live Arcade (2009)[note 1]
- Sega Fun Pack: Sonic Mega Collection Plus & Shadow the Hedgehog (2009)
- Sonic PC Collection (2009)
- PlaySEGA (2009)
- Sonic Classic Collection (2010)
- Virtual Console (2010)[note 1]
- Sega Ages: Sonic the Hedgehog 2 (2020)
- Though Sonic and Tails are replaced by Knuckles and locked by default, Tails can be seen at the end after the final boss.
- In the 2013 re-release of Sonic the Hedgehog 2, the color pallete changes to the act title cards and Shield Monitor from Knuckles in Sonic 2 were reverted to back to their original colors when playing as Knuckles.
- In Special Stages, Knuckles' name is not displayed like Sonic and Tails'.
- In the 2013 re-release of Sonic the Hedgehog 2 though, he is given the initials "K.T.E" in Special Stages.
- Knuckles arms are miscolored tan in the special stages.
- 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 to enable Debug Mode in Sonic the Hedgehog 2.
- The sprite of Knuckles that appears on the title screen is similar to the sprite that appears on the title screen of Sonic and Knuckles.