In Sonic the Hedgehog 2, it is confirmed that Knuckles is the last of the Echidna Tribe, a group whose main mission was to take back the Master Emerald that was stolen from them by the Owls.

and inform Sonic what it is that Knuckles wants from him. Sonic Learns His Destiny As The Protector Of The Master Emerald Knuckles mentions that he's looking for something called the Master ...