Classic Knuckles was the past counterpart of Knuckles the Echidna from Classic Sonic's world om the Sonic the Hedgehog series. He is also the guardian of the Master Emerald, a powerful ancient relic which is his solemn duty to protect from anyone seeking to abuse it.
Sonic the Hedgehog 3
Classic Knuckles is the original Knuckles who first appeared Sonic the Hedgehog 3 & Knuckles after Robotniks defeat at the hands of Sonic and Tails in Sonic the Hedgehog Robotnik's ship known as the Deathegg crashes in the jungles near Angel Island. It is here where Dr. Robotnik first encounters Knuckles who happens to not be very welcoming to the doctor knowing that it's only a matter of time before Sonic and Tails find him Robotnik manages to convince Knuckles to trust him and has his the echidna believe Sonic and Tails are the bad guys. Knuckles eventually catches on to Robotniks scheme and realizes the truth then together the three heroes stop the villain. Even after Knuckles realized that Sonic wasn't the one after his Master Emerald he still saw the hedgehog as his rival.
Sonic and Knuckles
Sonic Triple Trouble
Classic Knuckles also makes an appearance in Sonic Triple Trouble where he is tricked into help Robotnik making him believe Sonic and Tails are ater the Chaos Emeralds. As Knuckles helps Robotnik gather the Emeralds unknown to the four characters Nack the Weasel a treasure hunter who also goes by the nickname Fang hears everything and decides to claim the Chaos Emeralds for profit.