Little is yet known about the Master of the Hunt or his Hounds.
 
At the end of Mage: The Hero Discovered, the Hounds came up to Kevin in the Styx, wearing the faces of those who he knew had died.  They howled crazily and attacked, but Kevin was able to fend

them off physically fairly easily.  It was less easy for him to ignore their guilty message, but in the end he was able to lose his sense of guilt and they left.  The Master, who rides a motorcycle, stared at him before riding away.
 

In The Hero Discovered, the Hounds once again harassed him on two occasions, and he was able to beat them away.  This time, they took a different form: black wolf-like creatures, with no human faces, and firey eyes and mouths.  The first time they said nothing, the second time they spoke of his failings.  Both times, the Sprigginflints were with them, and it is possible that they were not the real Hounds, but merely creatures made to imitate them.
 
Later, they appeared in their normal forms in Kevin's darkest hour, along with their Master, and howled about his lost hopes.  The Master once again pointed and stared at him before leaving.  Kevin seemed to be able to rise above his guilt again.

It seems likely that this is building up to an important showdown in The Hero Denied.