Zack Busner is a recurring character in the fiction of British author Will Self, appearing in the short story collections The Quantity Theory of Insanity, Grey Area, Dr. Mukti and Other Tales of Woe, the novels Great Apes, The Book of Dave, and the 2010s novel trilogy Umbrella, Shark and Phone.
Because his previous work is referenced throughout the latter, it can be safely assumed that these events take place after "The Quantity Theory of Insanity".
He is shown to be able to not only accurately predict people's behaviour but to use this along with manipulation to ensure his preferred outcome.
In an interview with DNA India, Self described the similarities between Busner and himself: We both grew up in North London; we're both of Jewish heritage (although him more than me), we're both philosophically inclined, we're both highly sexual, we're both rebellious, we're both obsessed by mental illness (although him as a practitioner, me as a writer), we've both been married more than once and have numerous children, we both affect a certain dishabille, we both like Jimi Hendrix.
[1]Busner is a psychiatrist and psychoanalyst practising in London, and is prone to self-promotion at the expense of his patients.