Chad Halverson is an agent with the CIA.
Not that it matters a whole lot from the second page of the first book onwards. It plays a minor role, yes, but by and large, his former employment is moot. Everyone's former employment - heck, former anything, is moot.
Back in the States from one of his many assignments, Halverson was in the Langley headquarters when notified his brother had been injured in a car accident in L.A. Having not seen him in a couple of years due to his job, Halverson hopped a commercial jet and headed west. Landing, he and the other passengers found the world had changed.
A plague which had been hitting spots over the world had struck American soil and struck hard. It killed the host body but did not let that body stay truly dead. The brain remained somewhat operational but with one key motivation - food. The zombie apocalypse was at hand.
Throughout the series, the fight to stay alive against billions of reanimated ghouls is the key objective but when all order is destroyed and chaos takes over, the human inclination is to put some semblance of control back in place and that means numerous pockets of new governments rising. In these new enclaves, strangers are seldom welcome except as slaves as Halverson will find out several times.
Even as he, and a female companion he meets early in the second book and remains with, struggle to stay alive from the zombies and free from the various new orders, he also must evade the occasional drone attack from the little that remains of the former government and the Agency that learns he inadvertently discovered the truth about the plague's origins and now must die. [Why he must die is never really explained].
Halverson has his hands full from the moment that plane touches down. And they had better be filled with weapons.