Ren and Ai are twin first-year sisters at Kouta's school, and first appeared in the fifth volume of the novels. They are spirit hunters who live at a very poor temple, and are frequently seen doing things that homeless people would do, for example, making stew out of trash and roadside weeds, or eating thrown away bread crusts. Originally, they were given the task to assassinate Chizuru, but did not go through with it since she was still "only a kid". They are nearly identical except for their slightly different hair styles. They use chains when attacking.
Source: Wikipedia.