Ren Ichimoku, also known as Moku or Ishimoto Ren, acts as Ai Enma's companion throughout the series. His existence began as a sentient katana, a tsukumogami awakened after a century steeped in violence he witnessed but could not prevent, developing a deep aversion to bloodshed. Abandoned near a battlefield, he remained until Ai Enma discovered him. She offered him a new purpose, suggesting he join her search for an unspecified goal; he accepted her invitation, gaining human form.

In this human guise, he appears as a young man with chin-length black hair obscuring his unseen left eye. His attire evolves: initially a white and green hooded jacket over a black shirt with orange-tracked black pants; later a white and brown jacket with orange accents over a dark blue shirt; and finally a white cardigan over a purple shirt with brown pants. He consistently wears a distinctive square metal pendant around his neck. He displays vanity regarding his appearance and maintains a generally relaxed, smiling demeanor, though reacts strongly to human injustice. While considering Ai's other companions family, he often remains detached. He struggles to comprehend illogical human behavior and harbors a recurring dislike for Kikuri.

His abilities include projecting a large, disembodied eye onto surfaces to observe and listen remotely, even through obstacles; this eye can also emit intense light flashes and serves as a point for teleportation. He can alter clothing magically and manipulate memories to create illusions of familiarity. By kissing his pendant, he transforms into a dark blue straw doll with a red string. He travels freely between the human world and the Realm of Eternal Twilight.

Following Ai Enma's mortal death at the end of the second season, he lived briefly as a human, working as a sentō janitor. Kikuri later recruited him back to assist Ai's successor, Yuzuki Mikage; during this time, he posed as a science teacher at Yuzuki's school to investigate clients. In the third season, he and Hone Onna temporarily refused to aid Yuzuki as Hell Girl, instead investigating Tsugumi Shibata's departure from Saigawara City. They deduced Tsugumi left due to her failure to alter Yuzuki's fate. He resumed his role as Ai's companion after she reclaimed the Hell Girl position.