Walter Alston is a former priest and a half-demon, like his mentor Robert Vasic. Walter, rebelling against his mentors teachings, considers Robert to be his arch-nemesis. It is only Walter who feels this way, as Robert is only merely disappointed in his former pupil.
Rather than learning about demons through studies, Alston instead summons them. Something which he is quite good at. He is also always willing to summon any demon for someone should they ask, but for a price. He makes a good living from it, judging from his nice home.
In Spell Bound Savannah Levine and Adam Vasic break into Alstons home, only to discover he has been tortured and then murdered by two supernaturals who he had refused to summon a demon for. After speaking with Kimerion, a Demi-Demon under a Demon Duke in Adam's fathers court, they learn the demon they wished to summon was Lucifer. A Demon who cannot be summoned. Which may explain the visions Hope Adams had been having during her pregnancy.