Brent is a non-canonical character created for the 2017 Sony Pictures film Crooked House. He is a CIA agent and contact of Charles Hayward. Presumably Hayward met Brent during the time he worked for British intelligence in Cairo some years before the events in the show.
British intelligence suspected that Aristide Leonides was working for the CIA. One of Charles' first assignments in Cairo was to ascertain whether his grand-daughter Sophia Leonides was genuinely in Cairo as an employee of Sothebys as she claimed or whether she was also there at the behest of the CIA. After Charles returned to London and embarked on an investigation into the murder of Aristide, he call Brent for background information to ascertain whether Aristide's links to the CIA had any bearing on his death. Brent confirmed that Aristide was indeed a "friend" of the CIA. He helped the agency by financing anti-communist organisations during the Greek Civil War. In return, the CIA helped Leonides smooth over his "shady" dealings in Las Vegas.