Macer Gifford, 30, has been fighting on the front line with Syriac Military Council (MFS) in Syria for the last three years, assuming the roles of sniper and medic.
The UCL (University College London) graduate said: “ISIS were winning when I arrived, I joined the losing side.”
The public schoolboy, who snipes ISIS fanatics and provides medical support to his comrades, told the Daily Mail: “I won’t go back until ISIS has fallen. Only the death of a close family member would make me go back home.
“I’m fighting against ISIS. I’m helping the people here resist the fanaticism, the fascism, that has blighted their lives for so long.
“We have ISIS completely surrounded on all sides and we are squeezing them tighter and tighter.”
Macer is currently fighting to liberate ISIS stronghold, Raqqa.
The former city boy, who attended Kimbolton School in Huntingdon, said he feels “at peace” and won’t return home unless a close family member dies.
Since he joined MFS, Britain, the US and other countries have targeted the extremists with airstrikes.
Sometimes, he sleeps within 300 feet of ISIS militants and airstrikes have landed just over 500 feet away and sucked the air from people’s lungs.
Mr Gifford admitted earlier in the year that he had treated ISIS terrorists even though he would “be beheaded” if it was the “other way round”.
When working as a medic he said: “There was one ISIS fighter that came in last week that sums this war up.
“He was 14 years old and grievously wounded and we did everything we could to keep him alive.
“He didn’t look like a vicious ISIS fighter on his stretcher, he just looked like a kid.”
But despite being part of ISIS, Gifford added: “He was looked after, just one of our soldiers would be treated.
“That’s in direct contrast to the fate that I would face at the hands of ISIS.
“I despise ISIS more than anyone else – I’ve lost half my last unit to them.”