“It’s enough to make lesser people wilt,” Mr. Gidley said of the news media exposure focused on Ms. Hicks. “I made a joke that Britney Spears shaved her head and slapped a car with an umbrella over similar treatment from the paparazzi.”
Ms. Hicks’s proximity to the president has also been a matter of interest to those investigating the Trump campaign’s ties to Russian officials.
Investigators have sought to determine what she knew about the president’s decision to fire the F.B.I. director, James B. Comey. And in January, a former spokesman for Mr. Trump’s legal team had planned to tell investigators that Ms. Hicks had once said emails written by Donald Trump Jr. before a Trump Tower meeting — in which the younger Mr. Trump said he was eager to receive political dirt about Hillary Clinton from the Russians — would “never get out,” a claim her lawyer has forcefully denied.
Testifying in a closed session of the House Intelligence Committee weeks after Mr. Porter resigned from the White House, Ms. Hicks said that her job sometimes required her to tell “white lies” on behalf of the president.
Hours later, she announced she would leave the White House. Ms. Hicks, who avoids interviews with the news media, issued a statement saying she had “no words” to express her gratitude to the president. She had told her family and a few friends three days before that she planned to leave.
It had become clear to others that by the end of her tenure Ms. Hicks was exhausted by West Wing infighting and the daily grind of spending most of her waking hours on call to the president. It was a grueling lifestyle.
After waking up and often sending her first emails of the day around 4:30 a.m., Ms. Hicks would squeeze in a workout. Then, for much of the work day, she could usually be found in her closet-size office, where she would wait for the president’s inevitable call: “Hopester,” Mr. Trump would say, or “Hopey!” He sought her advice on any number of unfolding crises facing the White House, but also to check his instincts against hers.
Ms. Hicks never cultivated a life outside the White House.
Several current and former aides praised her political instincts and understanding of Mr. Trump’s messaging. She often drafted his public statements, adopting his tendency to speak in simple declaratives peppered with Trumpian favorites — “incredible,” “unmatched.”
From time to time, she advised him on whether an angry Twitter post he wanted to send would be in his best political interests. From time to time, according to a former White House official, she would tell him that it was.
Little is known about the degree to which Ms. Hicks was a mediating force in an atypically run White House with a president whose first impulse is often his final reaction.
While messaging on the tax overhaul was tightly controlled by a team overseen by Ms. Hicks, Mr. Trump has in recent weeks appeared to have little regard for any formal rollout of his administration’s policy goals. Instead, he has proposed the death penalty for drug dealers, entered into haphazard diplomacy talks with North Korea, and gone back and forth on his views on tariffs. And on Thursday, in an appearance in Ohio that was supposed to be about infrastructure, he seemed to stall a trade deal that had already been negotiated with South Korea.
There is a palpable worry among those in the West Wing about who the president will now confide in — and how many other people might be able to occasionally pull him back — now that Ms. Hicks is gone.
She is also among the people the staff relied on to bolster flagging morale — one White House official described her departure as a mother leaving her children behind. To cut the tension in a chaotic workplace, Ms. Hicks baked cookies for aides on Valentine’s Day, swapped country music song recommendations and texted her colleagues funny video clips.
Another person who has had conversations with both the president and Ms. Hicks in recent weeks, speaking on the condition of anonymity so as not to divulge private conversations, said that Mr. Trump relied on her and trusted her implicitly, mainly because she had never shown a willingness to put her own agenda ahead of his. She had also gained the trust of others in the Trump family for her loyalty.
“Hope is exceptional in every way and is loved by all who know her,” Ivanka Trump, the president’s daughter and senior adviser, said in a statement. “She is brilliant, dedicated and compassionate — a unique talent.”
The president’s chief of staff, John F. Kelly, has told several people he might leave the communications director role open for a time. But Tony Sayegh, a spokesman for the Treasury Department, is said to be up for consideration for the job, along with Mercedes Schlapp, the director of strategic communications. Another possible choice is Kellyanne Conway, who, crucially, is one of few senior aides able to communicate on Mr. Trump’s wavelength, as Ms. Hicks could.
For now, at least, the small office next to the Oval Office, a space reserved for a Trump confidante, will be filled by Dan Scavino, the director of social media and the only remaining Trump campaign original still in the White House. Before Ms. Hicks, that space was used by Keith Schiller, the president’s longtime bodyguard, when he worked in the West Wing.
Ms. Hicks, true to form, declined multiple requests to comment for this article. She has been coy with people who ask about where she will work next, or her plans for a vacation after a nonstop grind over three years, though she has told friends she is ready for a break. Regardless, she is likely to remain among those in the Trump alumni orbit, populated by other former White House and campaign aides the president tends to phone.
On her last day, Ms. Hicks had already sent 50 handwritten notes to senior staff and other people. In one of her last public appearances as an aide to the president, Ms. Hicks stood next to Mr. Trump as he emerged from the Oval Office and stood in front of news cameras for the first time in nearly a week.
He gave Ms. Hicks a kiss on the cheek, then set across the lawn, where Marine One was waiting. Two other White House aides, Stephen Miller, a senior policy adviser, and Ms. Trump, who was Ms. Hicks’s entree into the Trump orbit years ago, traveled with him to Ohio and then on to Florida.