In October 2016, the New York Observer asked a panel of real-estate moguls, including the paper’s proprietor, Jared Kushner, a simple question: Hillary or Donald? Kushner, who had married Donald Trump’s daughter Ivanka in 2009, responded with just two words – “Family first.” Now he has been named as a senior adviser to Trump, the incoming president of the United States.

As the candidate’s son-in-law, Kushner was a rare constant presence in the Trump campaign. On election night, after it became clear that Hillary Clinton would not win the White House, it was Kushner who took over his father-in-law’s mobile phone to screen the calls coming in for the new president-elect.

Jared Kushner was born in 1981 in New Jersey, the son of the real-estate developer and Democratic Party donor Charles Kushner. He went on to get a degree in sociology from Harvard in 2003. The investigative journalist Daniel Golden, in his 2006 book The Price of Admission, links a $2.5m donation that Charles Kushner made to Harvard University in 1998 with the acceptance of his son there the following year. Golden quotes a former official from Kushner’s school saying that his grades weren’t good enough for the Ivy League institution. (A spokeswoman for Kushner has denied this allegation.)

While at Harvard, Kushner made money buying and selling buildings, supported by his family’s fortune. His net worth is estimated at $200m. When he was 25, he bought the New York Observer for $10m, a move that helped him forge friendships with the likes of Rupert Murdoch. The paper’s print edition ceased production in November 2016.

In 2005, Charles Kushner was convicted on 18 counts of illegal campaign contributions, tax evasion and witness tampering. He served part of his two-year sentence in a federal prison in Montgomery, Alabama, where his wife and son visited him every week.

Jared Kushner’s favourite book is Alexandre Dumas’s The Count of Monte Cristo, in which the hero takes revenge on the enemies who wrongfully imprisoned him. For Kushner, it was the New Jersey governor Chris Christie whom he had in his sights. As a state attorney, Christie had prosecuted Kushner’s father. In December 2016, Christie was ousted as head of Trump’s transition team, reportedly at Kushner’s instigation.

Kushner practises Orthodox Judaism, and Ivanka Trump converted before their wedding. His grandparents escaped the Nazis, and his grandmother Rae was a founder of the United States Holocaust Memorial Museum. Kushner and his wife, who have three children, keep the Sabbath, although Kushner broke this during the campaign, attending a crisis meeting at Trump Tower on a Saturday when an audio tape emerged of Trump boasting about grabbing women “by the pussy”.

Trump’s chief strategist, Steve Bannon, the former executive chairman of Breitbart News, has been accused of allowing the website to run anti-Semitic articles. After allegations of anti-Semitism were made against Trump, Kushner wrote that his father-in-law was “an incredibly loving and tolerant person who has embraced my family and our Judaism since I began dating my wife”.

This support for Trump has come at a personal cost to Kushner. Friends from his liberal New York set have turned against him. In an interview with Forbes in December, he called this process an “exfoliation”. Kushner has said that going to rallies and meeting “ordinary Americans” helped him see Donald Trump’s appeal. At one rally, Trump spoke of his son-in-law’s new-found enthusiasm: “I actually think he likes politics more than he likes real estate.”

“Soft-spoken” is how Kushner is usually described, although a New York magazine profile claims that “his voice is just literally soft”, and sources say that his whispering conceals an aggressive manner. He is pale, slight and tall – a 2015 Vogue article said that both Kushner and his wife have “a kind of otherworldly, almost alien attractiveness, as if they’ve come from the future”.

In one sense, Kushner is the future. He is now a powerful figure in US politics, having met Boris Johnson, opened lines of communication with Binyamin Netanyahu in Israel and been instrumental in several Wall Street appointments to Trump’s cabinet.

Kushner and Ivanka are moving with their children to a $5.6m mansion in Washington, DC, so that he can work at the White House. It has been suggested that his new role contravenes a 1967 US federal law against nepotism in public appointments, but Kushner has said he will forgo a salary and is “committed to complying with federal ethics law”. Whether or not this law – originally passed in part because John F Kennedy nominated his brother Robert as attorney general – will prove to be an impediment remains to be seen.

What is certain is that Kushner’s fortune is built on his family’s real-estate empire, and now he has a job serving his father-in-law. Nepotism or not, his rise is a triumph of “family first”.