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.

"When we met Knuckles in Sonic 2, he had this singular focus to get the Master Emerald and protect it, because that's what his focus had been his entire life. This show asks what the warrior does ...