Who is Costello?
Robert Costello, a former federal prosecutor in New York, is relevant to Donald Trump's hush money case due to his role as a Michael Cohen antagonist and critic in the years since their professional relationship splintered.
Costello had offered to represent Cohen soon after the lawyer's hotel room, office and home were raided by the FBI in 2018 and as he faced a decision about whether to remain defiant in the face of a criminal investigation or to cooperate with investigators in hopes of securing more lenient treatment.
During Cohen's testimony last week, prosecutors presented correspondence showing that Costello boasted about his closeness at the time with Trump ally Rudy Giuliani - a relationship he suggested could be beneficial to Cohen - and reassured him that he was "loved" inside the White House. In presenting those messages to the jury, prosecutors hoped to prove that Costello's outreach was designed to keep Cohen in the Trump fold and to discourage him from flipping on Trump.
But Cohen ultimately did exactly that, pleading guilty to federal crimes and implicating Trump. Costello in the years since has repeatedly maligned Cohen's credibility and was even a witness before last year's grand jury that indicted Trump, offering testimony designed to undermine his account.