When Carrie Symonds was last seen in public, at the Commonwealth Day service at Westminster Abbey, her arm linked through Boris Johnson’s and beaming a wide smile, no one could have foreseen what drama the next seven weeks would hold.

From that day on 9 March to Wednesday’s surprise announcement that she had safely given birth to a “healthy baby boy”, the 32-year-old fiancee of the prime minister has endured an extraordinarily stressful time.

Separated from Johnson after he tested positive for Covid-19, she developed symptoms herself. She was self-isolating, with only their jack russell, Dilyn, for company, as he was taken to hospital. And, as he struggled in intensive care, she could do nothing except send texts and scans of their unborn son, willing him the strength to recover.

Reunited at Chequers on Johnson’s discharge from hospital on 12 April, the couple had been back at their Downing Street flat for just three days before the birth.

By any standards, it has been an emotional rollercoaster ride, and one which she will undoubtedly need time to process. So it would be surprising to see much of Symonds in the near future.

But then, she has kept a pretty low profile since she made news of her engagement and pregnancy public, posting on Instagram on 29 February: “Many of you already know but for my friends that still don’t, we got engaged at the end of last year … and we’ve got a baby hatching early summer. Feeling incredibly blessed.” Alongside was a photograph of her smiling, and Johnson kissing her cheek.

It was an unprecedented post, by a woman who has made history as the first unmarried partner of a prime minster to live in Downing Street officially in recent times.

The former Conservative HQ press officer, who rose to become communications chief, is seen as an incredible asset to Johnson. She is recognisable to young voters and her relationship with Johnson helps validate him among Tory millennials “hugely, absolutely” according to Sonia Purnell, the author of Just Boris: A Tale of Blond Ambition.

Johnson’s 25-year marriage to Marina Wheeler collapsed in 2018. His relationship with Symonds was reported in early 2019.

“She really has boosted his brand. Extraordinarily so,” said Purnell.

Her influence is said to extend from smartening up Johnson’s shambolic appearance, to augmenting the Conservative party’s green appeal. Michael Gove is among those to have realised her “green potential”.

She is no political ingenue. She has been steeped in Conservative party politics since joining as a press officer on graduating from Warwick University with a first in history of art and theatre studies. Though a Brexiter, her views are said to lie more with the progressive wing of the party. She is described as “socially liberal, greenish, economically to the right”.

She has worked as adviser to John Whittingdale at the culture department, to Sajid Javid at the Home Office, and to the former Tory MP Zac Goldsmith. She first worked with Johnson in 2012 on his successful second London mayoral campaign.

Her 30th birthday party two years ago was attended by heavyweights including Johnson, Javid and Gove. It came shortly after she and Johnson had been seen lunching on Valentine’s Day at Rules, in Covent Garden. With rumours swirling, their status as an item appeared confirmed when they were photographed exchanging glances outside a Tory fundraising ball.

She had, by then, left Conservative HQ to take up campaigning for Oceana, an eco-charity. She has been photographed with Johnson’s father, Stanley, at an anti-whaling protest.

“I think she’s unbelievably ambitious,” said Purnell, who said Symonds divided opinion.

“Some think she’s fun, she’s great, she’s introduced the whole green thing to a very dusty Tory agenda. She’s obviously clever and amusing. But a lot of really serious green people say she’s very good on cuddly, cute animals, but less so on tough decisions needed to make an impact on the economy, which she tends to shy away from.”

The daughter of Matthew Symonds, the co-founder of the Independent, and Josephine Mcaffee, a lawyer at the same newspaper, she was educated at the prestigious west London school Godolphin and Latymer.

She is well versed in the communications business, probably more so than her immediate female predecessors in No 10, Samantha Cameron, Sarah Brown and Cherie Blair. She was just 29 when she was appointed as Conservative communications chief. She was once ranked the second most powerful public relations operator in Britain by the trade magazine PR Week.

Carrie Symonds and Dilyn meet Lord Buckethead at the election count on 13 December 2019. Photograph: Will Oliver/EPA

She has an artful mastery of social media. At a time when she was the subject of unwelcome media coverage in the fallout from Johnson’s marriage break-up, she took to Instagram, posting: “Sea otters have the thickest fur of all animals.” Some have interpreted the message as evidence of her resilience.

Since her relationship with Johnson became public, Symonds has darkened her hair, dressed more modestly than in her more youthful social media pictures, and attempted to keep a low profile, mostly successfully. One exception was when neighbours called police to the flat the couple shared after hearing her shout “get off me” and “get out of my flat”, during what transpired to be a noisy domestic row.

When Johnson first got the keys to No 10, she stood alongside the rest of the Downing Street staff, though highly visible in a vibrant red and pink floral dress. After his December election victory, it was two months before she was seen by the press again, and then only leaving Downing Street by the back door. On 8 March, the couple were photographed in the stands at a Six Nations match at Twickenham.

Today, as the preoccupied mother of a newborn, she is again likely to remain largely out of sight. With the prime minister expected to delay paternity leave due to the coronavirus crisis, new motherhood is likely to be another, entirely different, emotional rollercoaster ride.