James Donovan is an agent with NESO.
I can find no reference to what that acronym stands for. It appears to be based out of the Pentagon which makes sense as Donovan is an active member of the military but beyond that, I have no idea.
As for Donovan, we do learn quite a bit about him early on. He is described as being "an ex-army ranger, who had for the past two years spent most of his time working for the National Security Agency, seconded from the Special Operations Unit of the Pentagon. But he did jobs which only a few operatives were cleared to perform. With a direct pipe-line to the President, he moved into White House grounds, interacted with CIA and FBI alike, and with only a no name picture badge ID: AAA-1A. He even entered security turnstiles with his weapon".
Further we are told that "most CIA/FBI and domestic officials knew that James Donovan was a smooth operator, who could easily compare favorably to MI6's James Bond". In appearance we know that "he was a good-looking man, who although turning forty in 3 months, was still handsome, strong and virile by anyone's standards. He felt that he was congenial, had some humor, in his own way, full of caring for the innocent and the elderly, and was known to give his last for someone else's benefit".
We also learn that Donovan holds the rank of Colonel in the military. He is said to have turned down a promotion to General likely because that would have forced his departure from the field.
Our first contact with 'Dono', as his friends call him, is clearing a bar that he apparently owns in Key West. His boss at NESO (or perhaps former boss), a former Pentagon strongman named Brady elected four years before to the Senate but still maintaining control of NESO, had directed Donovan to take a vacation until he was called for. That had been four months before and Donovan was more than anxious to be back into action.
In the course of the adventures we have of Donovan, his organization will change from NESO to one called CTU (I presume Counter-Terrorism Unit) but his close connection to both Senator Brady and the White House will remain extremely tight.
He will also meet and become extremely close to Magdalena Gonzales, a beautiful woman who goes from being the administrative assistant (and near god-daughter) to Brady to being pulled into the military with the rank of Captain to serve with Donovan. Their relationship will grow quickly resulting in their marriage soon after.