George Smiley is an agent with, and eventually spymaster of, the British Secret Service.
The history of Smiley is told as a foreword to the first book, letting the reader come to an understanding of the character far greater than you would get from most other series, all within the first few pages.
Smiley graduated from an 'unimpressive' school in his young twenties, trained in 17th century German literature and dreaming of a life as a Fellow continuing his studies. Life, in the form of a spymaster named Steed-Asprey, changes that direction when he approaches Smiley to help in the days just before WWII.
At first stunned at the suggestion, Smiley finally agrees and is sent to live in pre-war Germany. While teaching at a provincial German university, Smiley began to put together a spy network dedicated to giving his nation prior notice of how the German people were preparing for war. The fact that the leadership didn't listen did not change the value of the information he provided.
After subsequence servings in Sweden and Switzerland, Smiley was brought home to 'teach new men. Take time off. Get married or something.' He heeded his mentor's advice, proposing marriage to Steed-Asprey's secretary, the Lady Ann Sercomb.
His life as a spy was over, or so he thought. He had a new job teaching at Oxford and his marriage was a delight. Then Ann ran off to Cuba with a young lover in the first of her many infidelities and the Circus came calling. His services were needed again.
The best description of George Smiley comes from the first book. It reads that Smiley was 'short, fat, and of a quiet disposition, he appeared to spend a lot of money on really bad clothes, which hung about his squat frame like a skin on a shrunken toad.' It further refers to his 'fleshy, bespectacled face puckered in energetic concentration.'
In all matters, George Smiley is the absolute last person you would suspect of being a spy, least of all a man destined to become a great spymaster.
Unless, of course, it is your misfortune to have done something the Crown would did not like or you had some information that was thought important and Smiley was put onto it. His incredible persistance and tenacity, his inate understanding of people, albeit with considerable distaste, and his tremendously devious mind would wear you down and finish you. Smiley seldom lost because he could not envision himself losing.
For all that prowess at work, he is still a man that many underestimate, if only because it is better to belittle than to admit someone is so much better. Smiley is always considered "past it", even by himself at times. In Smiley's case it is likely weariness. His love of Ann has never wavered though he eventually says goodbye to her for good. His love of the Circus and his job is rewarded by being summarily swept out when a new broom came in and even the knowledge that he was begged to come and clean house himself later did not make up for the insult. He is left with his books which did not talk back to him and his memories which too often would not leave him be.