Denis Nayland Smith is an agent with British Intelligence.
Fu Manchu (often but certainly not always spelled Fu-Manchu) is the leader of the infamous Si-Fan international criminal organization and arch-enemy of Smith.
When we first meet the two, Smith was working as a "roving commissioner" in the British-controlled Burma. He had obviously been so for a while because his influence was considerable and his knowledge impressive. He had originally been working for the Colonial Police but eventually received this fiat to expand his area of operations. Some later analysts would claim, perhaps rightly so, that he was already working for British Intelligence when he began his "roving" while others have felt he would only later start working for them. Since his authority came from the Home Office, the differences seem small as the results were pretty much the same.
There is not a lot written about the man himself which is astonishing considering that the written accounts of his exploits would encumbrance more than a dozen full-length books and forty years of service for his Empire and against his nemesis, Fu Manchu. He is first described as a "tall, lean man, with his square-cut, clean-shaven face sunbaked to the hue of coffee." We know by inference that he is likely in his mid-30s at least to have attained his rank. We must assume that he is single for he never mentioned a wife, late or otherwise. Beyond the fact that he knew the man who chronicled the first three of the adventures, Dr. Petrie, from many years before, likely university but perhaps before that, we know nothing of his early years or how he got himself stationed in Burma. There is a whole lot of not much in Nayland Smith's life story.
The same cannot be said for his enemy, a man whose actions have resulted in his acclaim and his notoriety which far exceeds that of Smith, the man devoted to stopping his every foray into the West. Smith's initial description of Fu Manchu is "Imagine a person, tall, lean and feline, high-shouldered, with a brow like Shakespeare and a face like Satan, a close-shaven skull, and long, magnetic eyes of the true cat-green. Invest him with all the cruel cunning of an entire Eastern race, accumulated in one giant intellect, with all the resources of science past and present, with all the resources, if you will, of a wealthy government - which, however, already has denied all knowledge of his existence. Imagine that awful being, and you have a mental picture of Dr. Fu-Manchu, the yellow peril incarnate in one man."
The age of Fu Manchu is really anyone's guess. We learn over time that he has perfected an elixir which has greatly prolonged his life and kept him in obvious very good health. It is written that he has several doctorates from Western universities and is a genius in several fields with chemistry being a predominant one. As he has at least one full-grown daughter, as we learn over time, he must be a minimum of 40 years but likely, considering his influence, considerably more.
The Si-Fan organization with which he is affiliated is international in scope but largely Asian, spanning from India to northwest China but it is the region of southwest China that seems to draw him the most; Tibet to Burma. At first it would seem that Fu Manchu was but one of the leaders of Si-Fan for there are stories of animosity and out-right revolt in that group and there are even reports of Fu Manchu being considered a renegade with a price on his head from his colleagues. How true these might be is unknown but whatever discord that might have existed will eventually be quelled and Fu Manchu would become the unopposed head.
Nayland Smith (he seldom used his first name) would receive a Knighthood for his saving of London from Fu Manchu and would see his area of operations become global as he would operate across Europe, spend considerable time in the Middle East (especially Egypt), and then head West to the Americas where he would work closely with the FBI. In all this time there would be long periods where his enemy, Fu Manchu, would be quiet and unheard from and what Smith worked on then is never mentioned but each time Fu Manchu got active, Smith would be there to combat him.
Smith could never hope to match Fu Manchu in the brains department and he would several times admit quite honestly that his successes were largely a matter of luck and stubborn determination rather than strokes of genius. But succeed he would, over and over, though the ultimate prize of capturing and eliminating his opponent would always be beyond his reach.
In another 'envisioning' of the character, several years after the adventures recorded in book form had taken place, Nayland Smith would show up again, now an elderly but still determined man, to tell a young but extremely dangerous assassin named Shang-Chi the truth about the younger man's father and leader, Fu Manchu. Smith would convince Shang-Chi to join him in the seemingly unending struggle to keep the oriental villain from achieving his nefarious goals.
Amazingly, for all his accomplishments at fighting such a powerful foe and stopping that man's global reach, if you were to mention to the man on the street the name Nayland Smith, you would surely get a blank stare. Mention his enemy, Fu Manchu, though, and it would be a whole different reaction. Even here, though both are listed in the index, it is Fu Manchu that has the title. Certainly Smith never got a mustache named after him.