Philippe Reines, a longtime aide to Hillary Clinton and one of the most astute observers of her personal and political vulnerabilities, is playing Donald J. Trump in her mock debate sessions, according to people familiar with Mr. Reines’s involvement.

Mr. Reines, who was Mrs. Clinton’s chief defender, enforcer, and gatekeeper during most of her years in the Senate and as secretary of state, is a deft practitioner of the combative, no-holds-barred politics that Mr. Trump favors.

His selection as the Trump stand-in means that Mrs. Clinton wants an opponent in her mock debates who knows her flaws and how to exploit them and who is fearless about getting under her skin the way that Mr. Trump might at their first debate on Monday night.

He is also known as one of a small handful of members of her inner circle willing to be tough and blunt with Mrs. Clinton about her own missteps. Mrs. Clinton, in her memoir “Hard Choices,” described Mr. Reines as “passionate, loyal, and shrewd” and added, “I can always trust him to speak his mind.”