Malcolm Stewart was the chairman of the Brotherhood of Magicians. He was the father of Brenda.
Stewart arrived on the Braxiatel Collection after Marlow's death because Marlow had bequeathed his magic supplies to the Collection. According to the Brotherhood's by-laws, non-members were not allowed to learn the magicians' secrets, so Stewart intended to remove the artefacts, as well as Marlow's assistant Brenda, from the Collection. (PROSE: Nothing Up My Sleeve)