Mitch Herron is an agent with the Enclave.
In answer to the question of what is the Enclave, it is useful here to repeat a question asked by Herron roughly halfway through the second recorded adventure, "The Enclave> What the **** is the Enclave?" Obviously it is the same question but the salient point here is that Herron is the one doing the asking and he works for it!
Throughout the entire first adventure we know that the Herron is an assassin working for some organization; we just do not know what the name of that group is or anything else about it except that Herron was told to stand down from an assignment and chose for the greater good to disobey the order. Why he was told to go on the mission in the first place and then why he was told to leave it remains a bit of a mystery. Not so hidden is the fact that Herron kills people for them. We know this because he not only lets us know he is a paid killer, he keeps track of the number of eliminations he does along the way, the last one being #148.
What we do know from the beginning about his occupation is that "Death was Herron's business and business was good. He was a scalpel, used by the U.S. Government to slice out cancers." What we will not learn for a while yet, nor will Herron, is that the Enclave for whom he was doing these excising was itself a very nasty infection. When he learns that sad truth, his whole life and his reason for existing will change a lot.
We do not know much about Herron for the longest time and even then it is not much. He is likely in his mid to late 30s, single with no attachments (at least none he talks about or even thinks about).
We know that he is very good at killing and he also has a sense of humor which when coupled with the first part of that sentence is both interesting and a little morbid (okay, I enjoyed it!).
He is also a survivor which is good because a whole lot of very bad things are thrown at Herron, one right after another and sometimes on top of the other.