Gabriel Sey was born to a diplomat mother and an artist father. He spent his childhood traveling back and forth between Africa and America. In Africa, even at the age of 10, he often volunteered with the medical corps as a helper, fetching meds or tending minor wounds of those injured in what seemed like endless conflicts. He had a real knack for understanding the biology of the wounded and was steadily given more responsibility. He grew fascinated with the capabilities and limits of the human body.

The fact that his country was plunged deep into a colonial resource war didn't deter Gabriel from pursuing science. In fact, he actually used the opportunity to help people. He would wait for the fighting to pause, then go out to locate survivors and treat their wounds. Roaming the battlefield and using his unique—and occasionally questionable—methods to save the injured, he earned his nickname, "Trenchwalker." He was popular among soldiers for his medical knowledge, but many found his unceasing attempts at humor to be a bit too much.

He met Jason Miller Junior during this time and the two became friends despite their large age difference. Gabriel's efforts ultimately earned him a place as an unofficial part of Miller's squad. He eventually segued his way onto a Starfall research team when he became fascinated with the material and its effects on human biology. However, for reasons unknown to him, he was never allowed to work with the material itself and was restricted to purely theoretical work.

But the weight of his curiosity was overpowering, so he acquired some of the substance illegally and started to experiment on himself. He discovered that he was immune to its effects and saw the potential for its use, making him one of the first pioneers of Starfall to recognize the boundless possibilities it could unlock.

When the connection was established between a person's immunity to Starfall and their ability to create a Hunter, Trench promised himself he would do anything to unlock its full potential. Rumors circulate that he was involved in converting other humans into Hunters before becoming one himself.