Kevin Cecil is the dedicated steward of the Twining family, a role that his own family has held for generations. He is introduced as the sole servant who remains by the side of William Twining after the family fortune is lost, choosing to stay without pay out of loyalty rather than obligation. Despite the sudden collapse of William's wealthy lifestyle, Kevin calmly adapts by growing vegetables on the estate and continues to manage the household with an unshakable sense of duty.

In terms of personality, Kevin is generally level-headed, professional, and responsible, carrying himself with the quiet competence expected of an English butler. However, he possesses a distinct and somewhat contradictory vice: a love for gambling. He enjoys placing bets on horse races, house fights, and other wagers, a habit that often exasperates the pragmatic William. This quirk adds a layer of unexpected unpredictability to his otherwise composed demeanor.

Kevin serves a critical role in William's life that extends far beyond household management. He functions as a parental figure and surrogate family member, having cared for the young master since childhood, particularly following the death of William's parents. When the previous school chaplain, Ernest Crosby, is injured, Kevin takes over the position at William's academy, claiming the necessary religious qualifications from his mother's side. This positions him as a constant guardian, watching over William in both his home and academic life.

His key relationships are complex and built on layers of hidden truth. His bond with William is the most significant, defined by profound trust and emotional dependency; William considers him the only family he has left and is deeply hurt whenever Kevin acts distant. Kevin is also intensely protective of William, viewing the sudden appearance of demons like Dantalion with suspicion and irritation. This relationship is complicated further by the fact that the real Kevin Cecil left the Twining household years ago to become a lawyer. The Kevin seen throughout the story is actually the archangel Uriel, who assumed the butler's identity to monitor William, the inheritor of King Solomon's soul.

Regarding his abilities, Kevin is far more than a human servant. As the archangel Uriel, also known as the Angel of Repentance and Cruelty, he possesses immense spiritual power. He has only one wing, having had the other torn away by the archangel Michael as a form of punishment or control, and his continued existence is sustained by one of Michael's feathers. He commands extraordinary exorcising abilities and can create powerful barriers, such as the one he placed on William's back, which can suppress even powerful demons. He also has the authority to command subordinate angels, such as Raguel, to act on his behalf.

Throughout the narrative, Kevin experiences significant internal conflict. He is torn between his growing, genuine affection for William and his divine orders from Heaven. He was sent to observe the heir of Solomon, with the grim directive to perform "Ecstasy" on him if necessary—a process that would turn William into a mindless puppet for God. Initially depicted in flashbacks as a more arrogant and unforgiving angel who condemned King Solomon, his time living as Kevin Cecil softens him. When faced with the order to neutralize William, he ultimately finds himself unable to do so, choosing to follow his own will rather than the commands of Heaven. This decision marks his development from a being who blindly executed divine orders into an individual who chooses loyalty and affection over duty.