In the short story A Fairy in the Flat/A Pot of Tea, Lawrence St. Vincent is a young man, who contacts The International Detective Agency to find a missing girl, Janet Smith whom he is in love with.
After Tommy and Tuppence successfully conclude the case, he becomes a source of publicity for the agency in subsequent stories in the series. In the next story, for example, The Affair of the Pink Pearl, he is dining with the Kingston Bruces. A house guest loses a valuable pearl. Lawrence highly recommends the International Detective Agency and their special 24 hours service. In The Crackler, Inspector Marriot suggests that St. Vincent knows Major Laidlaw can could help the Beresfords get an entree into their social circle.
Lawrence is the nephew of and heir to the Earl of Cheriton.