A red echidna from Angel Island, and guardian of the Master Emerald. He was first introduced as an antagonist in Sonic the Hedgehog 3 and Sonic & Knuckles, having been told by Eggman that Sonic and Tails intended to steal the Master Emerald. Halfway through the game, Eggman betrays Knuckles and steals the Master Emerald himself, which forced Knuckles to accept Sonic & Tails help in getting it back. Afterward, Knuckles would then be one of the duo's chief allies in fighting Eggman, but his guardian duties keep him out of the action most of the time. Not much of his past was known, but that changed with Sonic Adventure which fleshed out his background quite a bit. Turns out, Knuckles is the last of the echindas after they were wiped out for attempting to use the the Emeralds and desecrating the shrine (and injuring numerous Chao) which the angered the at the time guardian Chaos. Afterward, the piece of the shrine became Angel Island.
If Sonic is like the wind, then Knuckles is like the mountain; stern and unmovable. So naturally, they tend to butt heads, but do get along occasionally. Because of spending most of his life on a secluded island, he naturally prefers to be by himself and has quite the wild temper. However, despite these traits he's a nice guy, and more than willing to abandon his post to help out his friends.