Gerald is the magician with a heart of gold. Okay, not quite. Not at all, really. As far as we know he's just as murderous as Black Arthur, but he's smarter in one important way: after observing the Ryves brothers from afar, he's able to recognize Nick's humanity even as his boss insists Nick is a demon through and through. Plus Gerald spares Mae and Jamie's lives, and he shows an obvious interest in Jamie's potential as a magician.
Though he isn't a major presence in The Demon's Lexicon, we suspect his role might grow in the sequel, The Demon's Covenant, and that he may be a harbinger of what's to come for young Jamie.