Sir James is a member of the triad involved in a conspiracy that is at the heart of The Merlin Conspiracy. He is the current owner, or heir to, a large estate called Castle Belmont, which is the site of a series of freshwater springs. In the garden containing those springs Sir James' companion Sybil Temple sets off a series of poisoning of magically talented members of the Royal Progress leading to their collective abductions, leaving the triad in power.
When he first appears in the novel, Sir James is teasing protagonist Roddy Arianrhod Hyde about whether or not her own magical lineage will help her to attract the new Merlin. This is at a point in the novel before the loss of the old Merlin, so he clearly knows something. Roddy doesn't know what he's talking about, but finds his leering looks offensive, so she changes the subject and leaves as soon as she can.