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 declare her Christian faith.

Four former hostages who shared cells with her, speaking publicly for the first time about their shared ordeal for ABC News’ “20/20” segment “The Girl Left Behind,” airing Friday, say Mueller — a 25-year-old humanitarian aid worker from Prescott, Arizona — was courageous and inspiring.

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 several times to a room next to one where male hostages were held. Former hostages said Emwazi paraded her in front of them to offer proof of life to the ones about to be released, having her remove her headscarf and briefly introduce 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.”