Agent 917 is an agent with The Agency.
What 'agency' that might be is not specified. It could be the CIA since 'the Agency' is often used to refer to that governmental department but this is only conjecture so we are left with just knowing that he works for some group inside the American government which has taken upon itself the role of eliminator of perceived threats to the status quo.
"I don't have a name, only a number - 917. I'm here to kill someone and when it's done, I can go back to sleep."
Those are the thoughts that are with this operative at the moment he awaken. Every time he awakens. And each of those times, he had absolutely no recollection of who he was and what he has done or why he is likely lying on an unknown bed in an unknown hotel in an unknown city. "Whoever he was before this morning was dead. Only two things remained inside of him: the duty to complete his next mission and his professional instincts."
As that last part indicates, he is not totally oblivious to what is happening. He does not recall what his true identity is but he does knows that this has happened before and that it is the Agency that has done this to him and that it has done so an unremembered number of times before. This knowledge brings with it a fair amount of peacefulness knowing that he does not need to worry or fret or pay heed to what went on before, only on what is to be done now. This awareness comes with it understanding the procedure, as shown in the first recorded adventure, that upon awakening fully dressed onto of a nondescript motel room bed, he "scooped up the car keys and stepped out into the parking lot" where he would use the fob to determine the car and then to open the trunk to find the equipment he was allocated. This included a cellphone from which he would receive his instructions.
The description we are given of this man is: "His brown hair was trimmed short at the sides and his face was freshly shaven. A scar curled around his right forearm like a question mark. He lifted his shirt and was unsurprised to see dozens of scars weaving up and down his ribs and back." Other than that small amount of information, we are left with the understanding that he is a fairly average appearing man likely so he can come and go without attracting too much attention - aside from the fact that the leaving part means some target is no longer breathing.