The master in "The Sisters Grimm" series is Mirror, which is revealed in book seven of the nine-part series, as of 2015. Mirror becomes a friend and confidant to the main protagonist in the series, Sabrina Grimm, after her arrival in the town of Ferryport Landing.
"The Sisters Grimm" series details the adventures of two sisters who become fairytale detectives after a tragedy leaves them in the care of their grandmother. The grandmother reveals to the girls that she is the sister of the Brothers Grimm, whose book of fables were actually case files that the girls must now investigate. The character, Mirror, is portrayed as the same mirror from the classic Snow White fairytale.