Knuckles is a red anthropomorphic echidna and one of the main characters in the Sonic X anime. As the last surviving member of the Knuckles Clan, he spent most of his life in seclusion on Angel Island, a floating island high in the sky, where he served as the devoted guardian of the Master Emerald, a powerful gemstone that controls the Chaos Emeralds. This solitary upbringing made him a loner and left him socially inexperienced, contributing to his honest but highly gullible nature. He is often tricked by Dr. Eggman into believing that Sonic and Tails are threats, leading to conflicts, though he eventually realizes the deception and works alongside them.

In Sonic X, Knuckles is described as wild, tough, and stubborn, with a serious and determined demeanor in battle. He is a master of martial arts and a skilled treasure hunter, relying on his immense physical strength to shatter rocks and overpower enemies. He can glide through the air using his dreadlock-like quills and climb walls using the spikes on his gloves, and unlike Sonic, he is able to swim. Despite his cool exterior, he has a short temper and is easily provoked, and he is notably shy around girls. He is intensely loyal to his duty and prefers to act on his own terms, as Sonic once remarked, "That's Knuckles. He does things his own way." His favorite food is grapes, and he dislikes bright, flashy environments.

Knuckles' primary motivation is the protection of the Master Emerald, a responsibility he inherited from his ancestors. After the Chaos Control event that transported him to Earth, he becomes dedicated to finding the Chaos Emeralds so he can return to Angel Island and resume his guardianship. In the first season of Sonic X, his appearances are limited, and he often acts as a rival to Sonic without being a true enemy. In the second season, his role becomes more active as he uses his strength to help locate the Chaos Emeralds and defend against Eggman's schemes. In the third season, which takes place in space, he plays a critical part in powering the spaceship Blue Typhoon by harnessing the Master Emerald's energy. This ultimately leads to the destruction of the Master Emerald when it is used to fuel the Sonic Overdrive cannon to defeat the Metarex, marking a significant turning point in his personal journey.

His relationships with other characters show a gradual development. He and Sonic start as frequent adversaries due to Eggman's manipulations but later develop a strong mutual understanding and respect, though Knuckles sometimes envies Sonic's carefree and adventurous lifestyle. He shares a playful, bickering dynamic with Rouge the Bat, especially in the third season, with hints of an unspoken romantic tension. He also appears to have a secret crush on Amy Rose, though this is not deeply explored. Despite his initial reluctance to rely on others, he grows into a reliable and heroic ally who is willing to sacrifice his most treasured possession for the greater good. His character embodies the contrast between the mountain, steadfast and anchored to duty, and the wind, free and ever-moving, and his journey in Sonic X highlights his struggle with that identity, ultimately leading him to embrace a broader sense of purpose beyond mere guardianship.