Hekate's Master is the person who is teaching her forbidden magics. He's currently unknown, although there was a thread on the forum where there were a number of guesses; it was a victim of the Forum Crash of 2011.
Hekate meets him in a cursed circle a mile outside of the school boundaries.
He warns the Alphas, via Hekate, not to prank Reach and Spark—not because he's interested in them personally, but because he has several sensitive projects in the Workshop and doesn't want additional attention from Security.
Hekate's Master talks to Don Sebastiano and gives him a message for the Necromancer. Sebastiano is convinced that Hekate's Master is a student. Sebastiano also later believes he has traced a phone call from Hekate's master to a male, non-wizard student, but no more information has been given to readers at this time, including about this information's reliability.
On May 1, 2007 Solange overheard The Don confronting Hekate's Master in the Tunnels. He claimed that he knew that Hekate's Master was known by The Syndicate as Nimbus. Hekate's Master neither confirmed or denied this.