He is a necromancer.
He lived in Coven Street, next door to the Chants, and made his living as a magic tutor. He charged one pound an hour for the Elementary Grades, and one guinea an hour for the Advanced Grades. Gossip has it that he sponges off his wealthier and more successful brother, Mr William Nostrum.
He is small and plumpish, with two wings of grizzled hair. He has a cast in his left eye, which always stares out sideways. He always talks to Cat in a jocular way and calls him "Young Chant" a lot, which Cat finds disconcerting.
See also Charmed Life (spoilers).