the relationship of the president and Hope Hicks, long tolerated as a quaint bond between the older man and a trustworthy young woman, began to be seen as anomalous and alarming. Completely devoted to accommodating him, she, his media facilitator, was the ultimate facilitator of unmediated behavior. His impulses and thoughts— unedited, unreviewed, unchallenged—not only passed through him, but, via Hicks, traveled out into the world without any other White House arbitration.

Her role (and Jared's) in crafting Trump's response to the NY Times disclosure of Don Jr's meeting with Russians while on Air Force returning from the G-20 summit, and in the firing of James Comey, has been of interest to Mueller's investigation.

Hicks was interviewed this week by the House Intelligence Committee for 8 hours, during which she refused to answer most questions about the Trump Administration asserting executive privilege.

Fire and Fury alleges:

Miller and Hicks had drafted—or at least typed—Kushner’s version of the first letter written at Bedminster to fire Comey. Hicks had joined with Kushner and his wife to draft on Air Force One the Trump-directed press release about Don Jr. and Kushner’s meeting with the Russians in Trump Tower.

So much of Fire and Fury comes from statements purportedly from Bannon, it's nowhere a certainty that the roles of Hicks and Kushner are accurately described.

The NY Times wrote a flattering profile of her here.

Trump aides say Hicks' departure was planned for a while. The Trump family is effusive in their praise for her. But with other aides close to Jared and Ivanka also announcing their departure this week (Josh Raffle, for one) I can't help but wonder, why is she really leaving?

Vanity Fair reports:

in recent weeks, Hicks became disillusioned with her job, people who’ve spoken with her told me. Her closeness to Trump meant she was also nearest to the hottest flames of the fires that have burned in the West Wing, the most intense being the Russia probe. Hicks has racked up substantial legal fees, one source told me. “She’s in immense personal jeopardy,” one Republican close to the White House said yesterday. “This is a sign the Mueller investigation is a lot more serious than any one of us thought.” (Hicks and the White House declined to comment.)

...The tipping point for Hicks was more personal, sources said. She was “emotionally overwhelmed” by seeing herself in the headlines during her boyfriend Rob Porter’s domestic abuse scandal, one Hicks friend told me. Hicks expressed disbelief at the allegations that Porter beat his ex-wives. She’s recently told people she’s no longer dating Porter.

Another factor, according to Vanity Fair, was her worsening relationship with Trump's Chief of Staff John Kelly.

Kelly pulled off a masterstroke by promulgating new security-clearance rules that neutralized his biggest West Wing rivals: Jared Kushner and Ivanka Trump. “The Jivanka team is crumbling,” a person close to the White House said. A day before Hicks resigned, Kushner’s spokesman Josh Raffel announced he, too, was quitting. Between the relentless chaos of the West Wing and the looming Mueller investigation, even the most committed staffers are re-evaluating their options.