Zac Miller is an analyst for the CIA.
Jake Keller is a field operative for the CIA.
Put like that, it would seem two men are involved in this series but there is just the one.
When we first meet this gentleman, he is Zac Miller, a "strategic weapons analyst" and is considered one of the best at what he does but he is decidedly not a field agent. When he volunteers to take the place of a blown agent on what Miller believes to be a very important op, he goes in knowing he is not trained and therefore not really ready but he believes he has a good chance of pulling it off. It goes very wrong.
So wrong, in fact, that when it all ends, it is necessary for Miller to disappear and so Zac Miller becomes Jake Keller. And Jake Keller has proven that he can survive really, really harsh conditions and come out on top and the CIA does not like missing such an opportunity.