Knuckles is a playable character in the Brawl mod Project+. He was revealed on March 17th, 2020 and was made available in P+ 2.0, which released on the same day. Knuckles is one of the three characters that were not originally playable in Brawl, with the other two being Mewtwo and Roy. Unlike Mewtwo and Roy, Knuckles is a brand new character rather than a Smash veteran.
Knuckles was one of few characters originally planned for Project M, but development of his character went into limbo after Project M ceased its own development following version 3.6. Knuckles is the first clone engine character from a third party universe, the third clone engine character overall, and the only newcomer to appear beyond what Project M intended.
As a possible callback to Sonic games made around the same time as Super Smash Bros. Brawl, James Snyder (credited as "Dan Green"), who voiced the character between 2005 and 2010 before being replaced by Travis Willingham, reprises his role through re-purposed voice clips taken directly from that era of Sonic games.
In competitive play
Z-Secret Costume: Cowboy Knuckles (Sonic the Hedgehog: The Movie)
R-Secret Costume: Sonic Adventure 2 gear Knuckles