Who is Lady Sybil Combrinck in the novel, Two Wrongs?

Lady Sybil Combrinck is the half-American, half-English Lady, that Bill has an affair with in London. When Bill shows up at her house for a party he has not been invited to, after the end of their affair, he is thrown out, and this begins the all-night bender that causes Bill to be absent when Emmy delivers her baby.