Satomi Murano

Shinichi's best friend and sometime companion. Shinichi and Satomi develop a romantic relationship over the course of the series, but their attraction to and fondness for one another is hampered by Shinichi's fear of his own monstrous nature, and Satomi's fear of Shinichi's secretive behavior and the mood swings that result from the trials and tribulations in his life.

Up until the last volume, she has no idea about Migi, until a near-death experience may have driven him to use his abilities without Shinichi's realization.
(Source: Wikipedia)