American hostage Kayla Mueller was tortured, verbally abused, forced into slave labor for ISIScommanders in Syria and raped by the group's top leader, but her fellow hostages say she never surrendered hope, she selflessly put the welfare of fellow captives above her own and she even stood up to executioner "Jihadi John" to defend her Christian faith.
Four former hostages who shared cells with Mueller, speaking publicly for the first time about their shared ordeal for ABC News' "20/20" broadcast, "The Girl Left Behind," airing Friday, say the Prescott, Arizona, humanitarian aid worker was a courageous 25-year-old who inspired them.
Their ISIS guards were overseen by the British tough Mohammed Emwazi, who would later be dubbed Jihadi John, as he carried out the beheadings and killings of 10 hostages. The Londoner led three other Britons who oversaw the hostage operation. Their prisoners called them "The Beatles."
In March 2014, Mueller was taken to a room next door several times where male hostages were being held. Former hostages said Emwazi paraded her in front of them to show prisoners about to be released who she was and to offer her own proof-of-life by removing her head scarf and briefly introducing herself.
Tune in to ABC News' "20/20" on Friday, Aug. 26, at 10 p.m. ET for the full Brian Ross report, "The Girl Left Behind."
VIDEO: The Kayla Mueller Proof-of-Life Video ISIS Sent Her ParentsPlay
The Kayla Mueller Proof-of-Life Video ISIS Sent Her Parents
Former hostage Daniel Rye Ottosen, a Danish freelance photographer, recalled how Mueller turned the tables on the men in black.
"One of the Beatles started to say, 'Oh, this is Kayla, and she has been held all by herself. And she is much stronger than you guys. And she's much smarter. She converted to Islam.' And then she was like, 'No, I didn't,'" Ottosen told ABC News.
He admits it surprised him a lot. He had once tried to strangle himself when ISIS guards strung his arms up by chains.
"I would not have had the guts to say that. I don't think so," he said. "It was very clear that all of us were impressed by the strength that she showed in front of us. That was very clear."
Former ISIS hostage Daniel Rye Ottosen is seen here in this undated photo taken while he was held captive.Daniel Rye Ottosen
Former ISIS hostage Daniel Rye Ottosen is seen here in this undated photo taken while he was held captive.
The only period in Mueller's 18 horrifying months as an ISIS hostage when she wasn't subjected to some form of torture, verbal abuse, prolonged isolation, sensory deprivation, stress positions, forced labor or sexual assault before she died in captivity was the six weeks she was held at an abandoned oil refinery in Syria, with other Western hostages in 2014.