Knuckles is a red anthropomorphic echidna, and portrayed as guardian of the Master Emerald. he was only seen briefly in Super Mario Bros. Z, in a flashback as one of Sonic's friends slain by Mecha Sonic (While not explicitly seen, the scene where Sonic explains his backstory implies that Knuckles was killed by Mecha Sonic riddling him with machine gun fire).
In the games, Knuckles and Sonic met during the events of Sonic 3. However Knuckles was tricked by Dr. Eggman into thinking Sonic and Tails were there to steal The Master Emerald, it wasn't until to late that he realised this was a distraction so Eggman could steal the emerald himself. They have had a friendly relationship since these events even teaming up in Sonic Heroes, however he is still shown to have a rivarly towards Sonic.