This should punch things up. #Knuckles https://t.co/RB4WY1qkrd
— Sonic the Hedgehog (@SonicMovie) August 10, 2021
The character first appeared in the third Sonic the Hedgehog game which was released on the Sega Megadrive (Genesis in North America) in 1994, he was the bad guy of the movie along with Dr Eggman.
Knuckles then became a good guy joining team Sonic in the fourth Sonic game Sonic & Knuckles which was also released in 1994.
No story has been announced for the new film which will get a release on April 22, 2022, also joining the movie is Sonics friend Two Tails.
Check out the Sonic the Hedgehog 2 page.