American fighting ISIS with Syrian Democratic Forces says he's on a mission for 'justice'

"Christian," as he calls himself, with his California accent, closely cropped blond hair, and tattoos that run across his face and neck, certainly stands out from the crowd in ISIS-controlled Syria.

He grew up in an orphanage in Sonoma, California, and received his military training in the French Foreign Legion.

Today, he's fighting with the Syrian Democratic Forces (SDF), a U.S.-backed group that includes Kurds, Arabs, ethnic Christian groups, Arab Muslim groups, Christian-religious forces and volunteers from around the world in an effort to topple ISIS in Raqqa, Syria, the de facto capital of the terror group's self-declared caliphate.

Christian keeps bullets marked with the names of places where infamous terror attacks perpetrated by ISIS occurred: "Manchester," "San Bernardino," "Orlando," and "Paris."

The names of these cities are printed along the sides of the cartridges he keeps in his bag.

He arrived in the Middle East in 2008, when he says he was first deployed, and developed friendships with many of the Yazidi and Kurdish people he met there.

After ISIS's rise in 2014, Christian learned that many of his friends were killed by the group, but he insists that his motive for returning to the region isn't simply a matter of enacting revenge.

"This isn’t really a mission of vengeance," he told ABC News about his reason for being in Syria, "but this is more like ... justice, you know?"

The Syrian Democratic Forces have been trained and equipped by the US-led coalition but do not have heavy weapons, tanks or armoured vehicles, things that were key for Iraqi forces to be able to defeat ISIS in Mosul.

Roughly 500 U.S. ground troops also fill out the region, although their presence is frequently shielded from the media's view; temporary forces in the region bring the total up to as many as 1,500.

That makes Christian's presence as an armed U.S. fighter in the region closer to the exception than the rule.

For Western volunteers like Christian who have dedicated themselves to fighting ISIS abroad, their work can sometimes earn them a minor degree of fame, like Brace Belden, a former punk rocker who was profiled in New York magazine after joining Kurdish fighters in Northern Syria and gaining a large following on social media.

The downside of the work, however, is its inherent danger.

David Taylor, a 25-year-old former Marine from Ocala, Florida, who traveled to Syria earlier this year to join a Kurdish militia unit was recently killed there, his father told The Associated Press this week.

Christian, for his part, is well aware of the inherent danger of his volunteer mission.

He says that bombs dropped by remote controlled drones are the biggest threat he currently faces from ISIS, and takes care to position himself in places where they can't get to him easily.

He describes these makeshift weapons as "an average $200 Amazon drone" that makes a high-pitched sound. The drones have an explosive attached to them that he says is "[big] enough to ruin your day."

Christian told ABC News that he sees hope in the unity of SDF soldiers, and admires the degree to which people of different faiths come together under a common cause through the group.

He said that education in the region, coupled with that sense of unity he feels while fighting for the SDF is the key to defeating ISIS long after the physical turf of their so-called caliphate is eventually reclaimed.

"I think the future in not just the Middle East but any conflict zone ... you have to educate the local people," he said. "I can kill 100 terrorists with these bullets, but with one book you can eradicate a whole city’s worth of terrorism."

ABC News' Ian Pannell, Lindsey Jacobson, and Matthew McGarry contributed to this report.