On a stage in a country town square, the accordion band struck up Edith Piaf's bitter-sweet love song, La Vie en Rose. François Hollande, just elected France's first Socialist president in 17 years, attempted a few steps of a waltz with his partner Valérie Trierweiler before she stepped back, perhaps realising they might look a little ridiculous on TV. "Kiss! Kiss!" demanded the crowd gathered in Tulle in Hollande's rural powerbase of Corrèze. It was Trierweiler who had chosen the Piaf song, stock soundtrack of France, and who had asked the mayor to play it.

The music marked the return of the accordion to French politics, not seen since the faux-rustic former president Valéry Giscard d'Estaing played it in the 1970s – an important message about Hollande's rural, Mr Normal image. But it also showed the subtle importance of Trierweiler behind the scenes. Moments earlier, the political journalist – who began a relationship with Hollande after years covering the Socialist party for Paris Match magazine – had sat with him as he put the last touches to his victory speech. Hollande might have won the election by styling himself as the Ordinary Guy, a powerful political branding exercise, but it was Trierweiler who appeared to have coined the term, having described him as "the Normal Man" in a profile she wrote in 2004.

Prickly, protective of her private life and not averse to calling journalists to chide them for what she claims are errors – such as reporting that Hollande dyed his hair – Trierweiler has been dubbed "Tweetweiler" for tweeting her disapproval whenever she feels wronged by the media. She is now under intense scrutiny as France's "first lady". The position doesn't officially exist in France or French protocol, but Sarkozy, who had two wives while he was in office – Cécilia Attias, followed by the former supermodel Carla Bruni – appears to have started a trend for partners appearing in the spotlight too.

If she keeps her job, Trierweiler could be revolutionary – the first president's partner to hold down a salaried post. However, she will also have to tread the minefield of being a journalist while being partner of the president. Polls have shown French people overwhelmingly want her to keep working, and approve of her independence. But Trierweiler has also been the target of attacks that show, even in the post-Dominique Strauss-Kahn era, that French political life is saturated with sexism. When Lionnel Luca, a rightwing MP from Sarkozy's party, called her "Rottweiler", and then added it was "unfair to the dog", Trierweiler said he should have been sanctioned by the head of parliament. A sports commentator was cut loose from his radio station this week for sending a tweet that read: "To all fellow women journalists: shag wisely, you could become the next first lady of France."

Hollande, 57, a moderate, jovial, moped-riding Joe Average, is so much the anti-Sarkozy character, so far the opposite of the bling, frenetic outgoing president, that Carla Bruni recently warned he could mean the death of French journalism: "If my man gets beaten, what are you going to write about?" she asked a reporter. Yet Hollande's personal life has been almost as complicated as the former president's and the subject of cover stories for years. The challenge now is how to handle it.

Trierweiler, 47, has been a political reporter for Paris Match for decades, covering the Socialists – not only Hollande, who lead the party for 11 years, but his former partner and mother of his four children, the politician Ségolène Royal. Hollande and Royal were the great political power couple of France, described as more competitive than the Clintons, though they never married because they saw it as too "bourgeois". Even in the 1980s, they began a blurring of the lines between private and public life, inviting cameras into their flat as their young children played or ate breakfast. In 1992, Royal, then a government minister, gave a controversial interview from a maternity ward, having just given birth to her youngest daughter. Trierweiler was the journalist who conducted it.

If this year's French presidential election has been defined by how loved-up the candidates were – Hollande and Trierweiler kissing and giggling, Sarkozy and Bruni doing the same – the last race in 2007 was dominated by relationship breakdowns. When Sarkozy was elected in 2007, his marriage to Cécilia was floundering, despite his attempts to suggest otherwise in front of the cameras. She would soon divorce him, and he recently blamed her for the worst of his showy excesses. His 2007 election-night party was at a flash Champs Elysées restaurant among the richest people in France, followed by a celebratory cruise on a billionaire's yacht. She has denied being responsible.

Royal, who ran against Sarkozy, was also in emotional agony. Hollande had left her for Trierweiler, a relationship that had begun in 2005 and had resulted in Trierweiler leaving her husband as well. But this was still officially a secret. Only after the election did Royal announce they had split and that she had asked Hollande to leave. But the rift in their relationship pervaded her campaign and came to be seen as a metaphor for the poisonous divisions running through the Socialist party.

François Hollande with his former partner, Ségolène Royal, 2007. Photograph: Xavier ROSSI/Gamma-Rapho via Getty Images

These conflicts threatened to open up again last year when Royal decided to run against Hollande for the Socialist ticket, but instead the former couple went out of their way to avoid confrontation on the hustings. After Hollande won, Royal became a key ally on the campaign trail and TV talkshows. To publicly end the rift, she appeared with Hollande at a rally in Rennes. Beforehand, Trierweiler approached her for a hand-shake for the cameras. Earlier she had confessed that she had not turned out to vote for Royal in the 2007 election.

Two years ago, the author Serge Raffy approached Hollande about writing his biography. It was at a time when Holland was in the political wilderness and was being seen as only a rank outsider for the presidency. Raffy says his interest in Holland was in part driven by curiosity at how intermeshed Hollande's private life was with his political one, but that Hollande had no concept of this. "I don't think he was aware of the storybook quality. He doesn't have that novelistic approach. I call him 'the Geometrist' – he sees political life as a series of equations."

If Hollande is to hold on to his image as dependable Mr Normal, he must keep all perception of his family and relationship life away from the excesses of the Sarkozy era. The final nail in Sarkozy's early popularity was when he paraded his Disneyland first date and Petra mini-break with Bruni, quickly making her his third wife only months after they had first met. His announcement at a formal presidential press conference in 2008, that "as you guessed, it's serious" with Bruni, went down so badly, he never gave another one.

Bruni, whose fortune is estimated at €18m (£14.4m), and whose love tokens to Sarkozy included a £45,000 watch, tried to downplay the bling during the recent campaign, saying they were "modest" people and revealing she travelled around Paris on the metro disguised in a wig. She once described herself as "the most apolitical animal possible", yet she is said to have influenced her husband on some areas, including the draconian Hadopi law on music and film piracy, and the appointment of her friend Frédéric Mitterand as culture minister.

Trierweiler, who says she feels as if she has walked through a looking-glass into the subject of one of her stories, claims to be better placed than Bruni to handle the workings of power. "I am and remain passionate about news. I know politics, I know the media. Bruni came from a world totally alien to that of politics. She didn't necessarily know the codes," Trierweiler commented recently.

Trierweiler goes out of her way to shake hands with a seated Ségolène Royal during a campaign meeting in April 2012. Photograph: Fred Dufour/AFP

This means that one line of attack Trierweiler has already had to dodge is that she influences Hollande, pulling strings. Journalists were surprised to find she had an office with her name on the door at Hollande's campaign headquarters. Comparisons were inevitably made with Sarkozy's first wife Cécilia, who advised her husband's political career and influenced appointments. Some began referring to Trierweiler as "the Duchess", always watching the manoeuvrings at court. She has admitted she has a strong personality and has refused to stop expressing herself on Twitter. "I thank my colleagues for respecting our private life and that of our neighbours. Please don't camp in front of our home," she tweeted last week. She also used her feed, @valtrier, to rail at her employers for sexism when she discovered herself on the cover of Paris Match, described as Hollande's "charming asset". During the campaign, the magazine trod a fine line, saying she was still working for them but not at editorial conferences or decision-making. Having previously hosted a TV politics chatshow, during the campaign Trierweiler switched to interviewing arts and sports figures. Paris Match says it will discuss her professional future after Hollande takes power today. She has already written her first lines about the campaign, captioning photos for a campaign book to be published in June.

Constance Vergara, a former colleague at Paris Match, is the only writer who Trierweiler has collaborated with for a book about herself which is titled, Valérie, Carla, Cécilia, Bernadette and the Others on the Campaign Trail. Vergara says Trierweiler bats off the Duchess tag. "She's very reserved, modest, quite shy and like many shy people, she can have an air of distance, coldness. That's how she protects herself. People think she's cold, contemptuous or haughty. In fact, I'd say it's the opposite; she's someone who laughs a lot. For 20 years she was in the role of observer and now she's become the object of observation. It's not easy to change status like that. She has already covered the G20, been to the White House, shaken Nelson Mandela's hand. It's not something that turns her head."

Vergara describes her as someone who always had an "element of class struggle". Trierweiler grew up one of six in western France in a council house in Angers. Her mother was a cashier at the ice-rink, her father lost a leg aged 12 while playing with an unexploded shell in the second world war. She refutes the "Cinderella" tag, pointing out that her grandfather was a banker, but she is still seen as a rebel, an outsider. She has always kept her job as a reporter: "She always refused promotion, wanted to keep her freedom," says Vergara.

Despite describing her as shy, Vergara concedes that, like Sarkozy and Bruni, Hollande-Trierweiler are prone to public displays of affection. He publicly calls her "the love of my life" and when she telephones, "My love" flashes up on his phone. "The kissing on the lips is new," Vergara says. "But both these couples are recent couples, second relationships, which flourished after 50. They're very much in love and it's on display. When Hollande won the Socialist primary race in October, it was the first time he kissed Valérie on the lips in public. She was really surprised and still hasn't got over it. For her, that really officialised the relationship."

After the ritz of Sarkozy and the right in power, Hollande is under intense scrutiny to prove that he lives a normal life, particularly as he is likely to have to call for financial sacrifices from the French people. He has already indicated that we would rather live with Trierweiler in their modest nondescript flat in Paris's 15th arrondissement with its Ikea furniture than the 370-room Elysée Palace with its private cinema and 900 staff, including white-gloved factotums who set the pendulums on its scores of gold clocks. He must avoid the shameless personal spending that has marked the right's two decades in power, particularly under Jacques Chirac. As mayor of Paris, Chirac and his wife's personal food bills came to £1.4m in eight years, including £40 a day on herbal tea. In some respects, Sarkozy cut the Elysée budget, in others – namely air travel, security and more than doubling his own salary – he was accused of raising it.

In some ways, Sarkozy broke taboos, on what constitutes a modern family for example. Hollande's inauguration will be the second time that a famille recomposée, or blended family, take possession of the Elysée. In 2007, Sarkozy put his second wife, two stepdaughters and three sons from different marriages centre stage at his inauguration. He leaves office with a third wife, a new baby daughter, another stepson and two grandchildren.

Hollande and Trierweiler are not married. He has four grown children with Royal; Trierweiler is twice divorced with three teenage sons. Royal has stated that none of her children will be present at the inauguration. "It's François Hollande who has been elected president, not a family, not his friends or his mates" she says.

Hollande's eldest son, Thomas, a lawyer who worked on his father's campaign, as he had on mother's, is the only child to have been in the public eye. On election night, the bearded and geeky 28-year-old wiped away tears of joy and took a phonecall from his dad live on TV just after his victory – the call coming up on his iPhone as "Papa". But, embarrassed by the attention, he has now retreated from the cameras. "He doesn't like talking about himself," Thomas has said of his father. "He's not very narcissistic." It's another sign that the Hollande family operation looks to be very different from the Sarkozys'.