Risa Kagami is the late wife of Shiranuma (now known as Byaku). Worried that her husband would lose interest in her if she only had beauty to offer, which would eventually fade, she became obsessed with becoming eternally beautiful, and took her own life after becoming disfigured. The demon Kagami assumes Risa's surname and form as part of a contract with Heisuke Matsudo.

Heisuke has feelings for Risa even tho she was married with Byaku and when she pass away Heisuke made an Ayakashi use her body to be his assistance.

(Source: kekkaishi.wikia)