Knuckles is a 16-year-old red anthropomorphic echidna, the only living descendant of a well-established clan of echidnas.[16] For many years, his clan guarded a giant gemstone called the Master Emerald, which controls the Chaos Emeralds, objects central to the Sonic the Hedgehog game series.[17] As such, Knuckles has spent most of his life atop a floating island called Angel Island, guarding the Emerald from harm.[18] He has grown up fairly introverted as a result.[16]

Knuckles has a serious, determined personality, described as keeping cool in combat.[19] However, he sometimes loses his composure and gets in fights with other characters, and is sometimes shy around girls.[20] His favorite food are grapes. His relationship with Sonic is ambivalent: Knuckles enters the series as an antagonist of Sonic and later sees him as both a friend and a rival,[21][22][23] while envying Sonic's adventurous lifestyle.[19] He is depicted as somewhat gullible and very rash, especially when angered.[17][24] He has often been tricked by Doctor Robotnik, but is stated in one game manual to have become more skeptical of him.[25]

While somewhat slower than Sonic, Knuckles is depicted as one of the series' strongest characters and a skilled martial artist: his brawny physique allows him to lift objects many times his size and weight, while his powerful fists enable him to smash through boulders.[16][17] He can also glide long distances by trapping air under his dreadlocks, and can climb up walls using the spikes on his gloves.[17] Unlike Sonic, Knuckles is able to swim.[22] As is typical among Sonic characters, Knuckles can roll into a ball to attack enemies.[26] He also has an empowered "super" form: using the Chaos Emeralds allows him to transform into Super Knuckles. Using the Super Emeralds allows him to transform into Hyper Knuckles.[27]