Page "Sonic & Knuckles" Paragraph 11

Referred to in-game as Knuckles the Echidna in Sonic the Hedgehog 2, the game is largely identical to Sonic the Hedgehog 2, with the exception that gameplay is altered due to Knuckles having separate abilities from Sonic or Tails.

Knuckles can glide and climb walls, allowing him to gain access to areas that had been otherwise hidden or unreachable.

Conversely, he cannot jump as high, making some situations, such as certain boss fights, especially the final one, more difficult.

However, in this version of the game, the two player mode has been removed.