Agent 10483 is an agent with the Organization.
Was an agent. He died. Ten years before the main part of the adventures begin.
His employer, the Organzation, was not unhappy about that ending. To their mind, while he performed excellently for them, by and large, he was a psychopath so having him no longer among the living and possibly causing troubles was a good thing.
We do not learn a lot of details about the Organization except that it is a government-funded security group which operates outside of the country, that is, Israel, and their purpose is to resolve current threats and potential threats to the nation by any means necessary. That usually means, as the expression goes, with extreme prejudice, i.e., death. Agent 10483 was especially good at that for them which is why they used him a lot over the years and why they so easily decided he was at heart a psychopath. How could one kill as easily as 10483 does and not be.
According to the journal that his counselor suggested he start using back when he was 10 years old, this psychopathy was nothing new and not something that came from his line of work. It showed to anyone able to read it that from an early age, 10483 had been "a complete nutcase" as one reader of the journal put it. It was why the Organization selected him and spent a good amount of time and effort training him. Honing a sharp weapon might be one good way to put it.
So why is the Organization as well as others not connected to that department interested in an operative without a known real name who has been dead for a decade? Well, there is the matter of the journal that has just resurfaced and which contains details on everything 10483 did for that group over the years.
That and the fact that this former government assassin, not really so dead as people thought, is now out to bring to an end yet another target - this time is the Organization itself.