An American hostage held by ISIS for 18 months who died in captivity is being remembered as a woman of courage and compassion who even defended her Christian faith to the executioner known as "Jihadi John."
ISIS abducted Kayla Mueller on August 4, 2013 while she was in Syria for humanitarian work.
"My name is Kayla Mueller. I need your help," an exhausted Mueller said in a "proof of life" video. "I've been here too long, and I've been very sick, and it's very terrifying here."
The 25-year-old endured torture, rape, verbal abuse, and slave labor at the hands of ISIS, but her fellow hostages tell ABC News she never gave up hope and put the needs of other hostages above her own.
Mohammed Emwazi, also known as "Jihadi John," supervised the ISIS guards. The man from London oversaw three other British men who were in charge of the hostages. Those in captivity called them "The Beatles."
Former male hostages explained that Jihadi John would walk Mueller in front of them to show those about to be freed who she was. It was also a chance for her to present an in-person "proof of life" by introducing herself.
A former hostage from Denmark, Daniel Rye, told ABC News how Mueller stood up to one of her captors.
"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,'" Rye said.
"I would not have had the guts to say that. I don't think so," he continued. "It was very clear that all of us were impressed by the strength that she showed in front of us. That was very clear."
ABC News reports that what is known about Mueller's time in captivity comes from the eyewitness accounts of fellow Western hostages and a Yazidi teenager who was also held captive by ISIS.
"She was amazing. She was a really strong girl," said fellow hostage Patricia Chavez, describing Mueller.
Another hostage, Frida Saide, from Sweden, told ABC News she had "a strong faith that gave her a lot of strength. As a person, she was a very good friend. She was smart, she was fun to be with. She was very kind, extremely generous."
"She was always considerate of others, even though she herself was in a very difficult situation," Saide continued. "She was always concerned for other prisoners."
"She never stopped being concerned for the Syrian population living through just horrible things in this war, and still are," Saide said. "She never stopped caring for others."
According to the ABC News report, Mueller's captors eventually handed her over to Abu Sayyaf, the oil and gas emir for ISIS, and his wife, Umm Sayyaf. The couple held Mueller and six Yazidi girls as sex slaves for Abu Bakr al-Baghdadi, the ISIS "caliph," the news agency reports.
Mueller's closest friend under the captivity of the Sayyafs in the fall of 2014 was a then-13-year-old Yazidi girl who requested that the network call her "Julia."
Julia told ABC's "20/20" that Mueller decided not to escape so her fellow hostages at the time, Yazidi teens, would have a better chance of making it to freedom when they fled late one night.
"I told Kayla we want to escape and I asked her to come with us. She told me, 'No, because I am American. if I escape with you, they will do everything to find us again,'" Julia said in the report, "'It is better for you to escape alone. I will stay here.'"
She also said Mueller told the girls that surviving involved being forced to act like she had converted to Islam so al-Baghdadi could sexually assault her, although she secretly refused to give up Christianity.
In addition, Julia said that Mueller prayed for the Yazidi teens to escape and survive.
"I will never forget this sacrifice. She was very good to us. I will never forget," she said.
ISIS said Mueller died in a Jordanian airstrike in Syria In February 2015. The Obama administration denied that, but a short time after ISIS made its claim, the White House said she died of unstated causes, according to ABC News.