The black sheep of a somewhat well off family, Nils was kicked out of his family home at the age of sixteen due to drug use and peddling, which was heavily frowned upon for a multitude of reasons. Instead of receiving any support, his family abandoned him and he got by the only way he knew how-- drug dealing and theft. For a time, it worked well enough but ultimately, inevitably he got himself into trouble. Lucky for him it was a first offense in a somewhat minor crime, and after that he attempted to clean up his act and get a "real" job, but it didn't last long and eventually he was back on the streets, doing what he did best.
By the time he's thirty years old in the year 2010, the infected have already destroyed the world he grew up in. He's barely getting by, mostly keeping to abandoned areas with drifers and other poor folks like himself. One unlucky day for him, he wandered into the wrong part of the town, and found himself attacked. He wasn't killed, and in fact he seemed to be okay after the attack but he had been envenomated. It took some time for him to feel anything apart from muscular pain where he'd been bitten and for a couple days, he thought maybe he hadn't been. A few weeks later, he knew he was wrong.
His time among the infected sprawled across the years. From 2010 to 2095-- eighty five years and he has barely aged a day. His mental health has declined significantly, as upon his "awakening" as what has been called a "Level V Patient Zero" by some, he has no recollection of the past decades. Very faint flashes of what he'd done as a Hunter and as a Reaper after that, but nothing cohesive to tie together. Nothing to explain how or why he can think for himself again, or why his blood lust has diminished so much.
It's troubling for him, and in his troubles he tends to find some solace in the familiar. During his life as a Reaper, he too envenomated a significant amount of humans and that seems to be where he finds his calm. Human food makes him ill, and though he doesn't need to hunt to feed, he tends to do so anyway if only for the thrill of the hunt that seems to make him feel more at ease. Having his humanity back is disturbing to him in a world that has clearly aged, and one he has no familiarity with.
His time with the Hearts gang has been minimal, and he operates more as a contractor for anything else. He's never been known for his loyalty and he dislikes the way the gang functions on a base level of why they protect the ones they're making money off of. In his eyes, humans are livestock, and he tends to treat them that way. As an associate, he finds the freedom to operate outside of their "rules" and has been running his own little racket outside of it as well, though he does this sporadically and only with the very rich as to avoid detection.
