Tarquin appears fairly soon in Enchanted Glass, approached by his brother-in-law Mr. Stock, who has an idea that his new employer, Andrew Brandon Hope, is in need of some help with the magical part of his inheritance.
So Mr. Stock goes to see Tarquin. He's a small man, with reddish hair and a tiny tuft of beard on the end of his chin, and he's missing most of one leg from a bad fall and trampling he'd had on his last day as a jockey. And he has a way with roses and a talent at cookery, as well as some kind of second sight that allows him to predict events.
Stockie's first suggestion, that Tark take Andrew aside and put a word in his ear, doesn't take well, but they both decide that his second, that they persuade Andrew that he needs a secretary, and persuade Tarquin's daughter (and Stockie's niece) Stache to apply for a job with him, just might work.
Tarquin's role in the novel is that of an ally, a helper, a wise person, and especially, an upright person.