To date Agent Coulson's origins have yet to be revealed. However, as he holds a position of command within S.H.I.E.L.D. thus we can assume that he has had some sort of military experience before joining.
Agent Coulson was created specifically for the 2008 live action motion picture, Iron Man. He had not made an actual comic book appearance until the release of an exclusive webcomic in April 2010, namely the digital comic "Iron Man 2: Phil Coulson: Agent of S.H.I.E.L.D. #1", later printed in actual comic-format. Although this is his first comicbook appearance, it would take another year for Agent Coulson to be established in the main Marvel universe. He first appeared (as "Cheese") in the 616 Main Marvel universe in Battle Scars Issue 1 (2011). It was only revealed in issue 6 of that series that "Cheese" was in fact Phil Coulson.
Mayor Story Arcs
Agent Coulson, as "Cheese", was a fellow Army Ranger and teammate of Marcus Johnson. He assisted Marcus as he was pursued by Taskmaster, Deadpool, Captain America, and Orion for unknown reasons. During these events, Marcus knocked out Coulson and left him behind to protect him. While separated, Marcus discovered that he was the son of Nick Fury. At this point Coulson contacted Captain America. Coulson's goal was to save his friend's life by enlisting Captain America, The Avengers, and SHIELD. The end of the series depicts Marcus, being called Nick Fury Jr., and Coulson joining S.H.I.E.L.D. and reporting to Director Daisy Johnson.
Phil Coulson and Nick Fury Jr. have been tasked to put together a new team of Avengers for secret covert operations. He firstly asks Hawkeye and Black Widow to join the team.
Digital comic: Iron Man 2: Phil Coulson
Agent Coulson's very first appearance outside the movies was in Iron Man 2: Phil Coulson: Agent of S.H.I.E.L.D. #1, witch build on the Iron Man 2 movie and is part of the 'movie universe'. It took another year or so before agent Coulson would appear in the regular Marvel Universe.
Actor Clark Gregg has portrayed Agent Coulson in all 4 of his appearances to date. His is contracted to continue playing the character in The Avengers & a likely Nick Fury movie.
When Agent Coulson appeared in the first Iron Man film, he was repeatedly ignored by both Tony Stark and Pepper Potts. Agent Coulson was meant to debrief Stark after escaping his kidnappers in Afghanistan, however his introduction was overly lengthy as he announced that he was from the Strategic Homeland Intervention, Enforcement and Logistics Division. He was a source of humour; providing a running joke throughout the film, which lead to the final revelation of the acronym " S.H.I.E.L.D.".
It wasn't until the movie was nearing it's climax that his character took a more serious turn. When Pepper Potts had taken secret files from Obadiah Stane's personal computer, she realised that he life was in danger. Therefore, the next time Agent Coulson approached her she latched on to him and insisted that he and his men accompany her immediately. Subsequently, Coulson lead a team of men into the Stark facility where the Iron Monger armor had been created. Agent Coulson clearly demonstrated his training as he lead his team silently through the premises and used weapons technology to break into secure areas.
His appearances in the sequel movie, Iron Man 2, were more limited. Agent Coulson didn't appear until after Iron Man had been in a drunken brawl with War Machine. Coulson aided Nick Fury in explaining the terms of Stark's House Arrest. Soon after, he announced that he had been called away to a situation that was developing in New Mexico. It was clear that Agent Coulson was a figure of authority within S.H.I.E.L.D..
In the post-credits scene to Iron Man 2, Agent Coulson arrived at the top of a crater to see several trucks and people gathered around something unique. It was revealed to be Thor's powerful hammer, Mjolnir. It was the first hint at the Thor movie. The scene was repeated within the movie, therefore insinuating that the events of both movies took place at the same time.
In the Thor movie, Agent Coulson is shown as a figure not only of authority, but of unquestionable political power, too. He repossesses the technology and data recorded by Jane Foster, but remains cool at all times and even offers monetary compensation (although it only serves to infuriate her further). He has the presence of a politician, while acting like a mysterious 'Man In Black' who casually steals someone' life's work on semi-regular basis.
Later, he orchestrates the security surrounding the Mjolnir crash site. This is most evident when he calls Agent Barton into action, but commands his movements via walkie-talkie. Instead of giving the order for Barton to shoot Thor as he tears through the complex, Coulson let the God of Thunder make his way to Mjolnir. This results in the spirit crushing scene in which Thor (who is still 'not worthy' to wield Mjolnir) can not lift the hammer & emotionally breaks down. Coulson calmly analyses his opponent and later interrogates him, determined to learn where Thor was training in several different fighting techniques. The interrogation was unsuccessful, but it was the first time that a darker side to Agent Coulson's personality was seen.
When Erik Selvig appeared claiming that Thor was really Donald Blake MD, Coulson's men immediately detected the fake ID. However, it was ultimately Coulson's decision to release 'Donald Blake'. Whether it was so they could further observe Thor in a more natural environment, or because he took pity on them is currently unclear.
The lighter side to Agent Coulson's personality is later restored as he comes to a tenuous agreement with Thor (after his immortal powers had been restored). Unintentionally, Coulson became the focus of a joke as Thor addresses him as "Son of Coul", based on the old Norse tradition the father's name as a son's surname.
Coulson also appears in the 2012 live action adaptation of the Avengers. During the movie it is shown Agent Coulson is a fan of Captain America, having a collection of Captain America trading cards. He was also present while Captain America was revived. After Loki breaks free on the Helicarrier, he tries to stop him with a blast gun but is back-stabbed by the god, and he apparently dies in Nick Fury's arms. It is hinted that Fury lied and that Agent Coulson is still alive somewhere.
Coulson appears as the undercover principal of Peter Parker's high school in 'Ultimate Spider-Man.'
Powers & Abilities
Agent of SHIELD
As a human being, Agent Coulson doesn't have any super powers. However, his position of authority within S.H.I.E.L.D. gives him political power to repossess belongings, break into weapons manufacturing facilities and wield firearms in public places. In the first Iron Man movie it was hinted at that he already had experience in covering up mysterious deaths (by providing a cover story for Stane's death), which was also something that Erik Selvig was concerned about when he recalled S.H.I.E.L.D.'s involvement in the disappearance of Dr Banner.
Just like most agents of S.H.I.E.L.D., Coulson will had have undergone rigorous miliraty training, is adept with a firearms and remains calm under pressure.