Hillary Clinton's 'Shrewd' Longtime Aide Playing Donald Trump in Mock Debates

— -- The mystery has been solved.

Hillary Clinton's longtime aide Philippe Reines has been chosen by the campaign to play the role of Donald Trump in mock debate sessions with the Democratic nominee ahead of the upcoming presidential debate, a source familiar with the debate prep told ABC News.

Reines' role, first reported by the New York Times on Saturday, has been a tightly guarded secret by the campaign. For weeks, reporters and politicos have been speculating over who had been tapped to play opposite Clinton in her mock debates, which is one of the ways she has been preparing for the first debate at Hofstra University in New York on Monday.

Watch the first presidential debate on ABC News and ABCNews.com at 9 p.m. ET. on Monday.

Clinton's aides had been looking for someone to play Trump who would not be afraid to rattle Clinton, as they expect the Republican nominee may attempt to do, or confront her about her husband's past sex scandals.

Reines' personality -- which Clinton described as "passionate, loyal, and shrewd" in her memoir "Hard Choices" -- is seen as a good fit for the part.

"I can always trust him to speak his mind," Clinton wrote in her book.

Reines, currently a political consultant, began working with Clinton more than 10 years ago when she was a senator from New York. He went on to become a senior adviser to Clinton at the State Department.

Although he has no formal or public role in her current campaign, it is known that Clinton still consults with him behind the scenes.

Clinton -- who is not new to campaign debates (Monday will be her 35th) -- has been preparing for her first debate against Trump for weeks. According to aides, she has been poring over briefing books at her home in Chappaqua, N.Y., studying footage of Trump's past debates, and practicing in mock debate sessions. Her campaign has also consulted with Tony Schwartz, the ghostwriter for Trump's "The Art of the Deal," on ways to get under his skin.

Much of her prep is focused on the unpredictable nature of Trump's personality, her campaign says.

"You're not sure who is going to show up," Clinton's communications director Jennifer Palmieri told reporters this week. "He may be aggressive or laid back."

Watch the first presidential debate at 9 p.m. ET. Full live coverage and analysis of the debate will begin on ABCNews.com/Live at 7 p.m. ET.