One of Mayor Cherelle Parker’s top aides is on the way out. But the ‘big three’ is here to stay.

The abrupt departure of one of Mayor Cherelle L. Parker’s top aides less than a year after she took office will be seen as a key moment in her administration. The question now is what kind of inflection point it will turn out to be.

Parker joined Chief Deputy Mayor Aren Platt, one of the mayor’s “big three” top advisers, in a City Hall news conference Friday to confirm that he was leaving city government on Oct. 25 and announce other appointments.

Neither offered specifics on why he was leaving or what he planned to do next. But Parker said he “is leaving on great terms with me and everyone in my administration.”

» READ MORE: Aren Platt, a top aide to Mayor Cherelle Parker, is resigning less than one year into her administration

”This is not punitive,” Parker said. “Aren Platt and I are going to continue working together... I will be excited to discuss the details of what that formally and officially looks like on another day at another time.”

Parker elevated City Hall veteran Vanessa Garrett-Harley to replace Platt as chief deputy mayor to round out the second iteration of her trio of top aides alongside Chief of Staff Tiffany W. Thurman and Chief Deputy Mayor Sinceré Harris.

As one of Parker’s most loyal advisers and a key architect of her victory in last year’s mayoral election, Platt’s resignation less than one year into Parker’s administration was unexpected. It’s highly unusual for people in the top level of an administration to leave so early in a mayor’s tenure, and neither the mayor nor Platt said what he will be doing next.

Kyle Darby, a City Hall lobbyist who was a policy adviser for Parker’s campaign last year, said Platt “has been a trusted and loyal adviser to her and is a big reason why she’s sitting on the second floor,” referring to the mayor’s office in City Hall. But he said it’s normal for new administrations to have growing pains, and suggested that whatever circumstances that led to Platt’s departure may only be a temporary setback.

”Unfortunately I think the timing is not the best, but all new administrations go through their issues,” Darby said. “There’s a lot that she’s undertaking right now that makes everything so magnified, but at the end of the day this is fairly normal. It’s never perfect when you first start out.”

On Friday, Parker and Platt were vague about why he was leaving city government, but the mayor effusively praised her longtime adviser.

“The portfolio of work that I have developed that put me in a position to say, ‘Look at this body of work, Philadelphia. This is how I delivered as a legislator. Now I want to be your executive’ — there has been no part of that work in my career that Aren Platt has not been a part of,” Parker said.

Platt said he was proud of the work he did in city government.

» READ MORE: Meet the staffers who ran Cherelle Parker’s historic campaign and will lead her mayoral administration

“Coming to work in City Hall ... I felt the enormity of my job,” he said. “Now it’s time to do what’s right for my family — being home for dinner and homework and cheering as my son runs cross country and my daughter plays soccer.”

Platt did not say what his next job will be, but said that “the ethics and integrity of this administration will be crucial” in whatever he does.

“In my future endeavors, I will ensure that I will be completely in compliance with all ethics and conflict-of-interest rules so there won’t even be the appearance of any impropriety,” Platt said.

Platt noted that he was leaving a position “that may be the best job I ever have.”

When Platt was asked why he was leaving the post, Parker stepped in and said: “He is going back to the private sector and a partnership and work that we will do in the future.”

“We will talk about that in the future at another more appropriate time,” she added.

Platt’s departure marks the end of Parker’s original big three. But the triumvirate structure is here to stay.

“This model of governance — it matters to me because it’s not modeled after any other mayor in the city of Philadelphia,” said Parker, who in January became the first woman to serve as the city’s mayor. “No one like me has ever served as mayor of the city of Philadelphia... I have to have a leadership and governance structure that complements my style.”

In December, when Parker laid out the structure of her incoming administration, she said she wanted to avoid a situation where there was one person who stood in for her.

“People were expecting to hear the name of the one person who was the big mahoff, so to speak. You know — ‘You want to get access to the mayor? You have to look to the big mahoff,’” Parker said at the time. “My mind doesn’t work that way. We have a team of people with very specific skill sets and talents who are equal.”

» READ MORE: Familiar faces and an unusual structure: How Cherelle Parker is shaping her mayoral administration

But an administration official, who spoke on the condition of anonymity because they were not allowed to speak to the press, said the three-person structure sometimes complicated decision-making.

“When you have three people try to make a decision, things can get bottlenecked,” the official said. “Some folks work around it. Some folks get frustrated both inside and outside of the city.”

Darby said that while the structure has “magnified” the attention paid to how Parker has handled some decisions, he doesn’t think it will be unworkable once the administration settles in.

“She has three people in place who have her ear, and she rules by committee because she’s loyal to her team,” Darby said. “I don’t think this is reflective of bad decision-making on the mayor’s part, truthfully. I think this is reflective of a new administration.”

On Friday, Parker was unwavering about her commitment.

“We are doing great things with this approach,” she said. “I am confident, very confident, of the direction of this administration and the governance model that I have developed.”

Now 10 months into her administration, Parker has taken an unusually long time to fill some key administration posts. The medical examiner’s office, the Department of Public Health, and the Department of Behavioral Health and Intellectual disAbility Services are all being led by interim leaders.

Key roles in planning and development, which fell under Platt’s portfolio, also lacked permanent appointees for months. But at the news conference on Platt’s departure, Parker announced several appointments for those positions.

» READ MORE: Mayor Parker’s development and planning team is finally taking shape 10 months into her term

Jessie Lawrence, who was Platt’s top deputy, will become the director of planning and development. John Mondlak, who was filling that role on an interim basis, will be Lawrence’s chief of staff. And Octavia Howell, a manager of the Pew Charitable Trust’s Philadelphia research center, will lead the City Planning Commission.

Parker also announced that Isabel McDevitt will become executive director of the administration’s community wellness and recovery centers initiative, a newly created position that is part of Parker’s effort to address the addiction crisis, including in Kensington.

» READ MORE: High profile departures leave Philly searching for a new slate of health leaders

Parker closed out the news conference with a message for the more than 30 administration officials who flanked her as she spoke in the Mayor’s Reception Room.

“Let me say publicly, I know it’s not easy working for somebody like me, and the reason it’s not easy because the people expect a little something different from me,” she said. “I do feel a little bit more pressure, and I’ll tell you all about why when I write the book about 50 years from now.”

Staff writer Jake Blumgart contributed to this article.