He grew up in an orphanage in Sonoma, California, and received his military training in the French Foreign Legion.
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.
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?"
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.
ABC News' Ian Pannell, Lindsey Jacobson, and Matthew McGarry contributed to this report.