Ren is Ai's second companion, and usually takes the form of a handsome young man in a green cardigan and a black shirt. His hair covers the left side of his face and that part of his face is usually seen. He has the ability to see the inside of a building through projecting the eye on the walls and ceilings. The large eye can also be used as a weapon through projecting intense flashes of light. When required, Ren becomes a blue straw doll. Ren's name means "one glance company". He is sometimes referred to as "Moku" or "Ishimoto Ren". His powers in human form is a reference to Daidarabotchi, a type of mythological giant that is enshrouded in shadow. It is later revealed that Ren is a tsukumogami, a type of spirit that originates from an artifact which has gained sentience after a long period of existence. In Ren's case, he was once a katana, forced to be aware and watch whatever was done with him. He was given his current form(s) by Ai, who collected him after he was abandoned on a large rock after a battle, during which his most recent owner was slain. Ai thinks that he is looking for something and asks him to accompany her. He agrees, saying that the rock he was abandoned on was getting pretty boring. Since that time, Ren has apparently grown quite fond of his human form, displaying considerable vanity from time to time. Ai claims she invited him to join her because there is something that Ren is looking for, a fact perhaps manifested in Ren's occasional puzzlement and inability to understand the things humans do. Alternatively, some of Ren's comments indicate that what he was looking for was companionship, something to fill the emptiness of his existence as a sword used for endless killing. Ren has apparently developed feelings for his colleagues, seeing them as family. Ren lives his life in peace as a human between the events of the second and third seasons. This proves to be short lived after he is recruited by Kikuri to become Ai's assistant again. Ren serves as a science teacher at Yuzuki's school several times while investigating clients that attend the school. At the end of the third season when Yuzuki becomes Hell Girl, he and Hone investigate why Tsugumi is moving out of town. They suspect it was because of Hajime and because Tsugumi was unable to save Yuzuki. Ren becomes Ai's assistant once again after she replaces Yuzuki as Hell Girl.