leaker Edward Snowden avoid extradition to the United States for exposing a massive surveillance programme.
Snowden and Harrison have been stuck together in the transit zone of a Moscow airport since the weekend, after she accompanied him on a flight from Hong Kong as part of efforts by anti-secrecy website WikiLeaks to help the American.
Harrison has been closely involved in Assange's own battle to avoid extradition to Sweden to face allegations of sexual assault, which he fears will lead to transfer to the United States and possible prosecution over his whistleblowing activities.
She virtually lived with the Australian WikiLeaks founder when he was under house arrest in the English countryside, and there have been reports that they were in a relationship.
Harrison now acts as a kind of gatekeeper at the Ecuadorian embassy in London where Assange sought asylum last year -- a move which personally cost her ?3,500 ($5,300, 4,100 euros) she had put up for his bail.
Friends said the researcher, believed to be in her late 20s, was an obvious choice to help Snowden.
"She's completely trusted," said Vaughan Smith, a video journalist who owns the house in Norfolk in eastern England where Assange lived under strict bail conditions between December 2010 and June 2012.
Harrison had her own room and was at the house most of the time, but Smith rejected suggestions she was simply Assange's adoring lackey.
"I don't think I ever saw her washing socks," Smith told AFP, referring to one newspaper report from the time.
"She's a key part of the team, she's one of the people who makes everything happen.
"She was very committed to the idea of greater government openness, very hard-working -- and put up with conditions of a very difficult job under great pressure."
Smith refused to comment on rumours that Harrison was Assange's girlfriend. "I know everything about it, but I'm not prepared to talk about that," he said.
Both are fiercely committed to their cause and Harrison appeared to complement Assange during a recent visit to the embassy by AFP, seeming organised and efficient where he is dreamy and remote, and helping him to find documents he has misplaced.
Harrison has worked for WikiLeaks since late 2010 as a researcher, media organiser and occasional spokeswoman, and is now Snowden's constant companion as he seeks refuge from US law.
A former WikiLeaks intern who asked not to be named described her as "formidable".
"Miss Harrison has courageously assisted Mr Snowden with his lawful departure from Hong Kong and is accompanying Mr Snowden in his passage to safety," WikiLeaks said of her role.
Questions have been raised about how a journalist with no formal legal training can help Snowden, but one of Harrison's former employers said she had unique insight into his situation.
"This is all new territory, when you're treated like some kind of enemy of the state," Gavin MacFadyen, director of the Centre for Investigative Journalism (CIJ), told AFP.
"WikiLeaks has been treated like that now for two years, and everybody who's been through it would have much more experience than [someone] who's never been there, hasn't heard the legal arguments, hasn't been subjected to the attacks."
Harrison shares Assange's well-documented concerns that WikiLeaks is under surveillance, making careful arrangements for encrypted communications with journalists about any of the group's projects.
She spent 18 months working for CIJ, located at City University London, sifting through WikiLeaks data and contributing to an as-yet-unpublished report on multinational corporations.
Harrison then worked for a couple of months at the Bureau of Investigative Journalism, also based at City, analysing classified US documents from the war in Iraq published by WikiLeaks.
"She came very highly recommended," recalled Rachel Oldroyd, deputy editor of the bureau. "She was very, very young, in her early 20s. She was a good researcher, very diligent."
Harrison's job is demanding and badly paid, but like everyone else working with WikiLeaks, money was never a factor. "She believes what she is doing is in the public interest," Smith said.