Jack Sinclair is a human character who appears in the television series Knuckles. He is portrayed as a former friend of Wade Whipple and serves as an antagonist in Wade's personal journey. Sinclair is introduced as the leader of a local bowling team called the Renegades, which Wade was a member of. During a crucial practice session, Wade fails to score a strike, leading Sinclair to publicly humiliate him by kicking him off the team and giving Wade's spot to a young girl named Susie Barnes. This event is the catalyst for Wade's subsequent adventure.

Sinclair's personality is characterized by arrogance, ruthlessness, and a flamboyant sense of self-importance. He views himself as a dominant figure, often referring to his team members as warriors, and he enforces a strict code of success. When Wade fails, Sinclair shows no loyalty, revealing that he never truly saw Wade as a friend but rather as a means to an end due to Wade being the son of a famous bowler. Later in the series, Sinclair falls on hard times and takes up work as a bounty hunter. He captures Wade, as well as Sonic and Tails, in an electrified cage, intending to turn them over to a mysterious figure known as The Buyer for a financial reward.

Despite his capture of Wade, Sinclair inadvertently provides the catalyst for his former friend's growth. Under the guidance of Knuckles, Wade enters a vision quest where he confronts his fears and learns the source of a warrior's true strength. Upon waking, Wade challenges Sinclair to a duel. The duel is an eccentric jousting match on motorcycles with swords. Wade ultimately wins by cutting off Sinclair's ponytail, a source of great pride for the bounty hunter, and knocking him from his bike. Following his defeat, Wade forces Sinclair to hand over his leather jacket and his prized cowboy hat, which Wade later gives to Knuckles. Humiliated, Sinclair flees the scene, and his fate beyond this point is not explored.

Physically, Sinclair is notable for his long hair, which he keeps tied back, and his distinctive cowboy hat. His abilities are not superhuman; his threat comes from his skill as a bounty hunter, his proficiency with a sword, and his willingness to betray former allies for personal gain. In Wade's vision quest, Sinclair is also portrayed metaphorically as the character Longclaw, serving as an antagonist in a rock-opera retelling of Knuckles's backstory. His key relationships are defined by his rivalry with Wade Whipple, whom he underestimates, and his transactional association with the Buyer.