Jack Renfield, first serving as Lord Rockwell's butler, possesses a formidable past as a British Army veteran of the first Afghan War. His battlefield prowess earned him the moniker "Jack," evoking a pocketknife, and later the feared title "Jack the Ripper" for his unmatched melee combat skill, making him dreaded by both allies and foes. He was hailed as the "first strength of the British Army" due to his unparalleled close-quarters abilities. Post-war, he returned to domestic service as Lord Rockwell's butler. In this role, he encountered the Moriarty brothers—William, Albert, and Louis—soon after the fire that consumed their family estate. Aware of his military history, the brothers specifically requested he instruct them in combat, an art he termed "the art of killing." He became their mentor in these lethal skills and subsequently joined their household as a trusted member of their inner circle. Standing notably tall at 6'6" (198 cm), he sports long gray hair typically tied in a ponytail and a goatee. During his initial association with the Moriartys, his hair was shorter, partially black with silver streaks. His attire evolved from a black waistcoat and white neck kerchief to a formal black three-piece suit after joining the Moriarty household. His personality reflects his butler background, displaying a consistently courteous and well-mannered, professional demeanor. Yet, he also reveals a humorous, teasing side, notably joking with associates like Sebastian Moran. He demonstrates unwavering dedication to William Moriarty's cause, adopting a serious attitude in critical moments, and occasionally exhibits flirtatious tendencies, such as his immediate interest in James Bonde regardless of Bonde's gender identity. A pivotal moment unfolded during the "Whitechapel Ghost" arc when imposters committed murders using his "Jack the Ripper" alias. Despite disliking the association, he agreed to impersonate the killer as part of the Moriarty group's strategy to expose the real culprits. He distracted authorities by posing as Jack the Ripper across London, buying crucial time for William to identify the conspirators. This operation culminated in the elimination of the imposters and the crisis's resolution, after which he expressed relief at severing the link to the moniker. In daily life, he retains habits from his service days, unconsciously cleaning or doing laundry despite no longer being a butler. He enjoys pancakes, values cleanliness, and dislikes insects.