Ren Ichimoku is a primary companion to Ai Enma throughout the series. Originating as a katana existing for over a century, he gained sentience as a tsukumogami by absorbing the suffering and violence inflicted during his use. Powerless to intervene, he witnessed extensive killing and cruelty, fostering a deep distaste for human conflict. His existence as a weapon ended when he was abandoned, stabbed into a rock near water after a battle. Ai Enma discovered him there, offering a new purpose: a human form in exchange for joining her service. This transition established him as her second companion, following Wanyuudou and preceding Hone Onna. His consistent human form features medium-length black hair covering his permanently concealed left eye. His visible eye is light olive, matching the color of his large projected surveillance eye. Attire varies by season: initially, a white and green hooded jacket over a black shirt with black track pants bearing orange stripes; in Two Mirrors, a white and brown jacket with orange accents over a dark blue shirt and black pants with gray stripes; in Three Vessels, a white cardigan over a purple shirt featuring light purple designs and brown pants. He always wears a distinctive square metal pendant, which he kisses before transforming into his dark blue straw doll form marked by a red neck string. Typically relaxed, he often smiles and engages in occasional humor. He reacts strongly to perceived human injustice, showing greater concern for sympathetic clients than other companions—a morality sometimes conflicting with his duties. He views Ai Enma, Wanyuudou, and Hone Onna as family yet frequently remains solitary despite interaction opportunities. He displays vanity about his human form and struggles to comprehend illogical human actions. Upon recruitment, he questioned whether joining Ai would aid his unresolved, undefined personal search. His responsibilities include investigating clients and targets with other companions. He uses aliases such as "Moku" or "Ichimokuren Ishimoto" during undercover operations to mask his non-human nature. During Ai's absence in Three Vessels, he worked as a sentou janitor before Yamawaro recruited him back, later serving as a science teacher at Yuzuki Mikage’s school. Near the season's end, he and Hone Onna declined to assist temporary Hell Girl Yuzuki Mikage, pursuing Tsugumi Shibata to investigate her reasons for leaving Saigawara City. He resumed duties with Ai after she reclaimed the Hell Girl role. His abilities encompass travel between the human world and the Realm of Eternal Twilight, magically altering clothing for investigations, manipulating memories to create familiarity illusions, and transforming into a straw doll. His signature power projects a large, disembodied eye onto surfaces, enabling him to see and hear distant events through obstacles, share these sensory inputs, and emit intense light flashes. This projection may represent his actual eye, as he closes his visible eye during use. He also demonstrates teleportation capabilities, potentially linked to his eye projection.