Avery is an agent with the CIA.
Early on in this series, we learn that Avery is what he is known by. I have not found (yet) any first name for the man since in typical military fashion it is common to call someone by his last name. I could be wrong.
I also know that he doesn't "do small talk" being thought of as "awkward and obstinate". I do not believe shyness plays a part but just that his nature is a solitary one. This loner-ism extends to his owning a ranch house in the "backwoods of West Virginia". "When there wasn't a job, he trained hard and stayed focused. He ran five miles four days a week. Each day, he targeted a different muscle group with weightlifting. Once a week, he practiced with firearms, either on the makeshift range in his backyard, or he'd make the drive to Quantico or the Point, where he'd also tackle the obstacle courses, the Kill House, or defensive driving courses to keep those skills sharp. Once a month, he'd make a day-trip rock climbing and hiking."
We also know that he had worked for some time with the Agency's paramilitary Special Activities Division (SAD) where his specialty was that of a 'cleaner' which in this context meant he salvaged blown or compromised ops overseas. "He'd quietly go in, remove the Agency's fingerprints from an embarrassing situation or mitigate the potential for blow-back, and slip back out." Further, we learn that his boss, Culler, was the go-to guy for the National Clandestine Service working for a group called the Global Response Staff. This division "provided independent contractors, recruited from the military and police SWAT units, to work undercover as bodyguards for case officers, do security at CIA bases and stations, and even operate as agent handlers and intelligence gatherers in high risk environments...The most lethal and proficient of these operatives are informally known as scorpions."
A very dangerous man is Avery. Perhaps it is good he likes to live alone.