What does a nuclear power with the fifth largest army in the world have to fear from a pint-sized university student in a pink frock? A great deal, apparently. On 31 January 2015, a North Korean government-run website posted an 18-minute video titled The Human Rights Propaganda Puppet, Yeon-mi Park, which denounced the charismatic 21-year-old defector. It was the latest attack in a smear campaign aimed at silencing Yeon-mi, a human rights activist and outspoken critic of the world’s most repressive and secretive regime.

Attacks on prominent North Korean defectors are nothing new. These individuals regularly endure charges that they lie and exaggerate. Occasionally there are death threats. Park Sang-hak, who launches helium balloons laden with USB sticks and anti-regime leaflets into North Korean airspace, has been called “human scum” who will “pay for his crimes in blood”. Sometimes the threats go beyond mere rhetoric: in 2011, a hitman with a poison-tipped needle was intercepted in South Korea on his way to kill Park Sang-hak. In 1997 the nephew of one of Kim Jong-il’s mistresses was gunned down outside Seoul; he had recently published an expose about the dictator’s family. But the regime’s most common weapon against its critics is character assassination.

“One of the very few growing industries in North Korea is this operation of trying to compromise defectors and witnesses,” says Greg Scarlatoiu, executive director of the US Committee for Human Rights in North Korea. The smears and threats have ramped up in the wake of a UN report documenting crimes against humanity in North Korea and recommending that the case be referred to the International Criminal Court. The UN findings were based on the testimony of more than 300 defectors who painted a picture of institutionalised cruelty within the regime, including mass incarceration in forced labour camps. North Korea has tried – unsuccessfully – to discredit the entire report because one of its well-known witnesses, Shin Dong-hyuk, later admitted to changing parts of his biography. “The fundamental building blocks of Shin’s story remain the same,” says Scarlatoiu. “He was still a political prisoner and still tortured.” But the controversy highlights a tragic catch-22: sometimes the traumatic firsthand experiences that make defectors such powerful witnesses also make them vulnerable to assaults on their credibility.

Yeon-mi did not testify before the UN inquiry, but became a YouTube sensation last autumn, following her emotional speech at the One Young World Summit in Dublin. Looking like a fragile porcelain doll dressed in a flowing pink hanbok (traditional Korean dress), Yeon-mi took the podium and, fighting to keep her composure, told a harrowing and heartbreaking story: “North Korea is an unimaginable country,” she began in halting English. “We aren’t free to sing, say, wear or think what we want.”

She said she believed the dictator could hear her thoughts, and she described the hideous punishments meted out to those who broke the rules or expressed doubt about the regime. When she was nine years old she saw her friend’s mother publicly executed for a minor infraction. When she was 13, she fled into China, only to see her mother raped by a human trafficker. Her father later died in China, where she buried his ashes in secret. “I couldn’t even cry,” she said. “I was afraid to be sent back to North Korea.”

Eventually Yeon-mi and her mother escaped into Mongolia by walking and crawling across the frozen Gobi desert, following the stars north to freedom. By the time Yeon-mi had finished with a plea to “shed light on the darkest place in the world”, the whole audience was in tears and on its feet.

Dissenting voice ... Park Sang-hak, releasing a balloon carrying anti-North Korea leaflets, has been branded ‘human scum’ by the regime. Photograph: Jung Yeon-je/AFP/Getty Images

After her Dublin speech, Yeon-mi became the human face of North Korea’s oppressed. The media clamoured for interviews. A book deal followed, which was where I came into the picture. As Yeon-mi’s “collaborator” – a publishing term for a writer who helps an author find her voice and turn her story into a narrative – I was immediately taken with the power of Yeon-mi’s testimony, as well as the warmth of her personality and her playful sense of humour. It was hard to fathom how this vibrant young woman could have suffered such an ordeal.

As soon as we began working together, I noticed there were some minor discrepancies in the articles written about Yeon-mi, a jumbling of dates and places and some inconsistent details about her family’s escape. Most of these issues could be explained by a language barrier – Yeon-mi was giving interviews in English before she was fully fluent. But Yeon-mi was also protecting a secret, something she had tried to bury and forget from the moment she arrived in South Korea at age 15: like tens of thousands of other refugees, Yeon-mi had been trafficked in China. In South Korea – and many other societies – admitting to such a “shameful” past would destroy her prospects for marriage and any sort of normal life.

She had hoped that by changing a few details about her escape she could avoid revealing the full story. But after she decided to plunge into human rights activism, she realised that without the whole truth, the story of her life would have no real power or meaning. She has apologised for any discrepancies in her public record, and is determined that her book be scrupulously accurate.

With Yeon-mi’s cooperation, I have been able to verify her story through family members and fellow defectors who knew her in North Korea and China. Sometimes Yeon-mi had forgotten or blocked out graphic details from her childhood, only to have the memories return in all their horror as we reviewed her recollections with other witnesses. It seemed that she wasn’t just remembering these things, but actually reliving them.

Countless scientific studies have shown that trauma changes how the brain processes memory. It turns out that scrambling details and confusing time frames is actually a sign that the trauma survivor is being truthful – they honestly can’t remember things in sequence. Dr Judith Herman, clinical professor of psychiatry at Harvard and author of Trauma and Recovery: The Aftermath of Violence from Domestic Abuse to Political Terror, tells me: “Traumatised people don’t give you a perfect, complete narrative on the first go-round. You see this all the time with refugees seeking asylum. That doesn’t mean their story isn’t credible, because the gist of their story is consistent.”

‘One of the very few growing industries in North Korea is this operation of trying to compromise defectors and witnesses’ ... the leader, Kim Yong-un and military chiefs. Photograph: Yao Dawei/AP Photo/Xinhua

According to Herman, more and more judges in US asylum hearings understand and accept the impact of trauma on memory. Unfortunately the general public can be less forgiving. Which is why North Korea, its sympathisers and dupes have been able to exploit the discrepancies in some defectors’ stories as a weapon to attack their motives.

A few days before Christmas last year I checked my inbox and found a mysterious email warning me not to write the book with Yeon-mi and help “spread her lies”. Then came the North Korean video. Despite having the production values of a Stalin-era newsreel, the footage was chilling: some of her uncles, aunts and cousins still living in North Korea were paraded in front of the camera to denounce her. The worst they could come up with was that Yeon-mi was an ambitious child. But it was horrifying for her to see them so vulnerable. At least she knew they were still alive.

Yeon-mi continues to work on her book and to speak out for freedom in North Korea. On Human Rights Day last 10 December, Yeon-mi appeared on a panel at the US State Department with another outspoken defector named Joseph Kim. In many public forums, Kim has told how he watched his father wither and die from starvation, how his sister disappeared and his mother left home, and how he survived as a street kid, scrabbling for crusts of bread. “Hunger is humiliation,” he says. “Hunger is helplessness.” But on this day he added something new: his mother had sold his sister to a man in China, thinking it was better than sending her back to North Korea.

“This is an important part of my story,” said Kim, “that I hope illustrates how difficult and desperate the life is, and how many North Korean mothers were forced to make this kind of heartbreaking decision.”

Does this fresh revelation in any way diminish this remarkable young man’s credibility? Absolutely not. It only shows how traumatic narratives sometimes come out in fits and starts. The whole story does not emerge until the survivor finds a way to tell it. But for those who have the patience to listen closely, the stories gather and build to a heroic and truthful testimony of survival that cannot be silenced.

• Author and journalist Maryanne Vollers has collaborated on several memoirs including Living History by Hillary Clinton. In Order To Live: A North Korean Girl’s Journey to Freedom, by Yeon-mi Park, will be published by Fig Tree/Penguin in the UK and Penguin Press in the US in September 2015