William Catesby served Richard III with unwavering loyalty from the moment of Richard's birth. He stood tall with a knight's physique, possessing blue eyes, dark brown hair, and dark skin implying foreign heritage. Characteristically quiet and observant, his minimal speech often left him at a situational disadvantage. Yet, his protective instincts toward Richard drove his actions with focused efficiency.

Catesby held intimate knowledge of Richard's intersex biology, a closely guarded secret central to Richard's struggles. During Richard's childhood, Catesby actively managed the concealment of Richard's heterochromia by styling his hair to cover one eye. This role as confidant and protector forged a deep, enduring bond. Though technically reassigned to serve Lord Hastings after Edward Plantagenet's battle with Warwick, Catesby's primary allegiance remained steadfastly with Richard. He prioritized Richard's emotional well-being, deliberately withholding information about Henry VI's identity as the supposed killer of Richard's father to spare him anguish.

His loyalty sometimes manifested as possessiveness, particularly concerning Richard's relationships. He displayed initial hostility toward the Earl of Buckingham, which intensified when Buckingham questioned the nature of Catesby's bond with Richard. Despite this friction, both men shared the goal of supporting Richard's claim to the throne, leading Catesby to set aside personal animosity for strategic cooperation. While Catesby's devotion was primarily platonic, it developed into unspoken romantic yearning over time. He consciously suppressed these feelings, believing he could never serve as Richard's "light," choosing instead to maintain his role as a steadfast, silent supporter.

Catesby readily pledged allegiance to Richard following Buckingham's public declaration of Richard as king, endorsing his political ambitions. His loyalty persisted until his canonical fate: capture and execution by forces loyal to Richmond three days after the Battle of Bosworth.