Her voice low, her posture tense, the woman who spent years steering Donald J. Trump through strife and scandal stepped to the witness stand on Friday carrying a different burden. She was there under the fluorescent lights of a dreary Manhattan courtroom, seated 15 feet from the former president she once fiercely defended, to testify at his criminal trial.

“I’m really nervous,” Hope Hicks, the onetime Trump spokeswoman, messaging maestro and all-around adviser, acknowledged to the prosecutor questioning her, declaring what was already obvious to the riveted courtroom.

Ms. Hicks’s unease came to a head hours later as Mr. Trump’s lawyer began to cross-examine her — and she began to cry As her voice cracked, Mr. Trump locked his eyes on her.

The question that initially unnerved Ms. Hicks was about her time at the Trump Organization, the family’s business, where she had fond memories of working. Ms. Hicks left the stand, and the trial paused so that she could compose herself. She returned minutes later to continue her testimony, occasionally dabbing her eyes with a tissue.

The striking show of emotion reflected Ms. Hicks’s discomfort with testifying against a man who launched her career and entrusted her with his reputation. Each time the questioning conjured up another memory of working for Mr. Trump — at his company, on his campaign and finally in his White House — Ms. Hicks appeared to fight back tears.

Ms. Hicks, who fell out of favor with Mr. Trump once it emerged that she had privately voiced anger at the Jan. 6, 2021, attack on the Capitol by his supporters, said in her testimony that they had not spoken in nearly two years.