It's Anthony B-B-B-Blanche
. There are several theories as to who the actual model for the figure was, but Harold Acton seems to be in the lead.
EVELYN WAUGH AND BRIAN HOWARD
Robert Murray Davis (University of Oklahoma)
"In A Little Learning, Evelyn Waugh announced that "the characters in my novels often wrongly identified with Harold Acton were to a great extent drawn from" Brian Howard, another would-be arbiter of society and art at Oxford and later in London (...). With the publication of Brian Howard: Portrait of a Failure, edited by Marie-Jacqueline Lancaster (...), Waugh's readers can now appreciate the extent to which Johnnie Hoop of Vile Bodies as well as Ambrose Silk of Put Out More Flags and "Basil Seal Rides Again" and Anthony Blanche of Brideshead Revisited were caricatures or portraits of Howard.
Because Howard was an inveterate social climber and self-styled leader of the Bright Young People, Waugh's first cut at him as Johnnie Hoop must have been felt most deeply. Hoop and Archie Schwert, another climber, are taken in by Adam's hoax in his "Mr. Chatterbox" column about block suede evening shoes, and Hoop's mother, like her son athirst for modernity and elegance, is one of those drawn to the tube station mentioned in the column as a fashionable literary haunt."