**(This first section is based off both headcanon and what facts we are told about Kim in the first movie. Everything beyond is of events that occur in the first three films.)**
Philby was born in India in 1911 to John Philby, a diplomat for the British government at the time. His full birth name was Harold Adrian Russell Philby, how he got the nickname 'Kim' was from his father; who was a huge fan of Kipling's novel 'KIM' about an Irish-Indian boy who spied for the British. Philby loved India and his father was very much into immersion into culture, while his wife (Kim's mother) was incredibly English and therefore...not so keen on letting her boy run rampant through the streets and forests with other children. Instead of attending a public or even private school, he was educated at the estate his parents lived in. Philby often went on extended camping trips in the forest with his father where they would fish, climb trees and also where Philby was introduced to Scotch. As he got older, he became more rebellious (as teenage boys are wont to do). He discovered girls no longer had cooties and how to ride a motorcycle. Naturally his mother was horrified. She and his father agreed to send him to England for furthering his education at Westminister in London when he was 14. Two years later he left. Soon after he won a scholarship to Trinity College, Cambridge where he studied history and economics.
The first movie takes off at Kim Philby's last year of college. After attending a party, where he met Guy Burgess, it becomes clear that Philby was/is highly in favor of communism (as were many Cambridge men during the time) and he believed that one must be either a fascist or a communist: 'Anything else is just appeasement'. He was best friends with a young Scotsman named Donald Maclean whos father was the Cabinet Minister. With Philby's father being as well known of an advisor in Asian affairs as he was, and Maclean's in such a well placed political seat they were prime targets to be made spies for the NKVD. Guy Burgess and Anthony Blunt, two Apostle men at the college began taking a marked interest in the two students. Kim quickly realized that they were being recruited for the Communists to work for them. Kim begins to associate more with Guy and Anthony and quickly became attached to them. Kim graduated from Cambridge with the other three (Maclean, Blunt, and Burgess) with a 2:1 degree in economics. After the graduation ceremony Donald and Kim agree that 'this is the beginning. No going back.'
Soon after graduation, Anthony introduced Kim to their handler named Otto. Kim was given an assignment in Vienna to aid refugees from Nazi Germany, there he met a young woman (and communist) named Litzi Friedmann or 'Alice'. He acted as a courier to Vienna and Prague, paying for the train tickets out of his own pocket (of which he didn't have much) and using his British passport to evade suspicion. He also delivered clothes and money to refugees from the Nazis. Kim grew attached very quickly to Litzi.
After spending a few months in Vienna, Kim fell in love with Litzi and asked her to marry him so they could use his British passport to get her out of Vienna before the Nazis could hurt her. Very much in love and very happy, Kim introduced her to Blunt and Burgess once they arrived. It became quickly apparent to Anthony that in order to maintain their appearances as 'right wing' Kim could not be married to Litzi - who was a known communist and Jew.
Kim is forced to unceremoniously tell her he doesn't love her and divorces her in order to continue his double life.
In the vein of becoming 'more right wing' Kim and Guy Burgess renounced their communism publicly and joined the Anglo-German Fellowship, a pro-Nazi pressure group. They both began to openly express right-wing opinions in order to provide a cover and to 'bury the past'. At this same point in time, Julian Bell (whom Guy Burgess was infatuated with) went to Spain to drive ambulances for the Republicans instead of fighting. His parents thought he'd be safer but Julian died nevertheless.
After this, Kim became a reporter for The Times and was sent to Spain to report on the war to England. A couple of years passed and he provided articles that were very sympathetic to Franco and the Nationalists despite Kim's true opinions. He encountered the war in small increments at first. But then on a fateful day, Kim and three other journalists piled into a car and went to a town looking for news. Instead they were bombed by a German plane and their car flipped, trapping all four in the vehicle until help arrived. Kim and one other journalist were the only ones to survive, the other journalist was hospitalized for months while Kim walked away with nothing worse than bruises and small cuts.
Guy then went to Spain some time after, and Kim informed him that Franco would personally give him a medal for 'outstanding reporting' - specifically the Red Cross of Military Merit. He neglected to tell him that the car accident is the reason for the medal (To protect Guy). Guy then revealed that he was sent to Spain to assign Kim a mission: Kill Franco. They both agreed it must be done for the greater good and Guy gives him the gun he's to shoot Franco with.
Kim goes to Franco, is fully prepared to shoot him and die in the process of doing so. But the fact that it would ruin his friends and their lives for being connected to Franco's assassin was too horrible of a thought that he didn't go through with the assassination. Franco pinned the medal to his chest, stood for photos, and lived to kill another day. Kim walked back to his flat, angry and saddened with himself. Upon arriving Guy tells him that 'if I had to chose between my friends and my country, I should hope I have the guts to betray my country. Friendship, Kim. Friendship above everything. England, France, Spain - doesn't matter.'
Kim soon returns to England.
These events convinced those that were right-wing in British politics that Philby had abandoned his former views on communism and the world. Guy Burgess then suggested to the chief of the Conservative Party and the chief of staff of M15 that she should recruit Philby. She agreed and Kim was given security clearance by Guy Burgess.
Kim is then informed by his handler, Otto, that his actions in Spain (not killing Franco) have had consequences and as such, Otto would be sent back to Russia. Which meant his death. With nothing to be done about it, Kim continues his work under his new handler, Henri.
After attending a piano concert with Anthony Blunt, Kim and the rest of the four spies hear the news that Germany and Russian have formed an alliance. Russia is now the enemy. They are now traitors in war-time and as such, can be killed if they're found out.
This ups the stakes dramatically and has a disturbing sobering effect on all four men. Of course, this being a time for seriousness, they get drunk and proceed to stay awake till morning.
Good times.
Donald then goes to France for his work in the Foreign office and meets his future wife, a woman made Melinda, and smuggles her back into England before Germany invades. Kim becomes head of Soviet Intelligence in M15 which means he has total control over who is exposed and who is not. Donald returns, happy for once, and Anthony and Kim agree that this makes him unreliable. Guy being deeply unpredictable, Anthony confides in Kim that he's been summoned for vetting. With Anthony worried that they found a trace from him to Russia, Kim gives him a few tips on how best to pass the questioning. Anthony gets through no problem; it turns out that he was being vetted for his services to the Windsor family in their art collection.
With that worry over came another problem, in the form of James Jesus Angleton. New to the spy game, James was assigned to shadow Kim and see how they do things in England. Angleton was (at this point) easy enough to handle and trip up according to Kim and Angleton was kept in the dark about Kim's double spy-life successfully.
Another wrench in the machine popped up though. Donald revealed to Melinda that he spied for Russia. This jeopardized everything and everyone. The skittles had begun to fall but they'd do their damnedest to stay upright.
Things started to look up when M15 decoded a message that revealed (before the public knows) that Germany was to invade Russia. They were 'double patriots', as Guy said jokingly. With Russia once again an ally of England, they all had a new peace with what they did.
All except Anthony. He couldn't take any more of the spying, his nerves completely shot and he felt that it would be best if he only worked for the Royal Family, still spying for Russia but not as a double agent.
But he couldn't in all good conscience leave the circle without providing his friends with some protection. He acquired some incriminating letters from the Royal Family and kept them as a blackmail fallback. Kim was both devastated and furious that Anthony would leave him in M15 alone with Donald and Guy to look after.
And that is where the Skittles are taken from.