Richard Braun, aka the Fifth Corner, is an agent with the Corporation.
First, about that Corporation, we learn that it is a secret organization of highly trained and very deadly operatives privately owned and run by an extremely intelligent and dangerous billionaire named Dr. Grandwell. Braun's job for this company is both as an agent and an assassin, doing whatever is necessary to accomplish the mission regardless of who gets hurt. Considering that he has been working for the Corporation for quite some time by the time we meet him, there are undoubtedly a lot of hurt, and killed, people.
Regarding Dr. Grandwell, reviewer Michael Shonk for Myster*File had this great line: "[He is] a man who never lets a life or country get in his way of making money". That clearly shows what kind of a man Grandwell is and by extension, what sort of company the Corporation is.
As we will learn quickly on, the name I put down for him, i.e., Braun, may or may not be his real name. Considering the number of passports he has with different names but the same image of him, he has a lot of identities from which to choose. Is Braun his real one? Don't know! Does he remember what his real name is? Same answer.
One of the reason he might not remember who he really is is because he does not remember much of anything. As the narrator tells us at the very beginning:
"He woke up and couldn't remember who he was, where he'd been, or what he'd done. And then he discovered he was not just one man but many, a spy with many talents; an operative for a billionaire's secret organization that wants him back. His only chance to survive is a woman who holds the key to his past ... and knows the secrets of his soul. So he's running for his life, trying to discover who he is and why we call him 'the Fifth Corner'.
So we have Braun not knowing who he is and not really liking the little he is learning about himself and we have the Corporation still wanting him to work for them despite his apparent walking away and we have a person or two inside the Corporation who have major issues with Braun causing them to believe their lives would be much better if his came to an end.
While Grandwell might want Braun's continued service, that billionaire's chief enforcer, a man with the quite odd nickname of 'The Hat' wants Braun dead - dead - dead! And Grandwell sort of gives him permission to try and make it happen, apparently amused by the conflict.
By the way, the reason for the almost equally strange name of the series and the main character, 'the Fifth Corner', is that he has an amazing ability to get out of extremely tight spots, as in 'if there is no way out, he will find one'.