While Agent Phil Coulson is undoubtedly the main character of the show and the reason for its existence in the first place. Unless the character returns in a future MCU film, the series finale marks the end of his story and the last time the character would appear in the franchise.
Each new season has brought new elements to the character, evolving him into the person he is at the very end by changing both the way the audience views him and how he views himself.
He Learned The Truth About TAHITI
Coulson's death at the hands of Loki was a moment that shocked a lot of fans of the character and they were equally as shocked when it was revealed that he survived the attack and would be getting a series.
For most of season 1, Agent Coulson was under the impression that he was able to pull through and recover and that he then went on vacation to Tahiti. It wouldn't be until later that he would uncover the truth and learn that he was actually the subject of project TAHITI, an invasive procedure that used Kree DNA to revive him and then alter his memories by operating on his brain while he was still awake.
He Became Director
At the end of season 1, after faking his own death and going underground, Nick Fury put Coulson in charge of SHIELD and officially tasked him with rebuilding the organization from the ground up, making him the new Director.
Under his leadership, the organization was almost able to completely wipe out HYDRA and win the war against the Inhumans. He was also at the forefront of dealing with the Inhuman outbreak and was responsible for investing in ways to provide them with ways to understand and utilize their abilities.
He Retired
The season finale showed the team each going their separate ways, bring an end to the journey Coulson had started them on. Some of the of the group remained at SHIELD like Daisy and Yo-Yo, while others retired from the agency like Fitz and Simmons. Like the latter, Coulson was also retired but still retained his connections to the agency.
Throughout the season, it was unclear if he would deactivate himself due to him coming back as an LMD but, when asked after a year, he says that he needed more time to decide and would to travel the world in the meantime, giving the character a much-needed and well-deserved vacation.
He Started A Relationship With May
Coulson has been in several romantic relationships on the show but the most popular among fans has to be the one between him and his long-time partner and fellow agent, Melinda May.
Coulson had initially recruited to pilot the team's plane in the first season but the pair's true feelings about each other came during the LMD arc of season 4 when they were both replaced by artificial versions of themselves. After the reunited and got out the framework, the two decided to explore a relationship together with May briefly retiring when he did and staying with him until he died.
He Lost An Arm
In season 2, after a fight with Gordon in which he was killed by Fitz, Coulson grabbed a Terrigen crystal to stop it from shattering on the ground and killing the Mack and Fitz. However, since Coulson was not Inhuman, the crystal began to turn him to stone.
Thinking quickly, Mack chopped off his arm with an ax and he was later outfitted with an enhanced prosthetic created by Fitz. The new arm had a ton of features and abilities that would later aid him in combat and would serve as a physical reminder of the cost of the job.
He Died...Again
After Coulson made a deal with Ghost Rider to help take down AIDA, it was revealed in season 5 that the side effects of having the spirit of vengeance possess him meant that the Kree serum that saved his life would be burned out and his heart would eventually just stop beating and he would die.
After the team completed their mission of keeping the Earth from being cracked apart and before they set out to go find Fitz in space, Coulson said his goodbyes and would spend his last days in Tahiti with Agent May until his death during what was intended to be the original series finale before 2 more seasons were ordered.
He's No Longer Director
Season 4 saw a lot of shifting roles and leadership among the team, including Coulson losing the position of Director and being replaced by Jeffery Mace. Coulson would later regain the position but then due to him dying it would be ed on again with Agent Mack was nominated as a director by the others in his place, although his first choice for the job was Daisy.
In the final season, even though he didn't have the official title, he would still serve as the leader of the group in spirit with Mack often turning to him for advice.
He's an LMD
At the end of season 6, it was revealed that in an attempt to help the team with their new mission of traveling through time and stopping the Chronicoms, Simmons had created an LMD of Coulson complete with his memories and mannerisms and upgraded with Chronicom hardware.
This major change for Coulson led to asking a lot of existential questions about his existence like what it really meant to be human and if he had a soul, further developing the character in a really interesting and unique way.
He Has Superstrength
Coulson has always been a skilled fighter but when he received his new Chronicom-LMD body, his new form granted him extreme durability and superstrength, letting him easily fight against and beat opponents that would have given him trouble before.
This allowed him to hold his own against opponents like the other Chronicoms and even Inhumans like Garret and Nathaniel. The enhanced strength he now had was something that he and those around him would have to adjust to, with both Daisy and Simmons stating that they were going to have to get used to him having it.
He Can Speak Code
Another skill Coulson gained in season 7 was the ability to read and understand code due to him spending a year in the 1980s stuck inside a television after he destroyed the Chronicom ship and himself in the process.
Thanks to this new ability, he was able to translate the code that appeared on screen when Sybil was communicating with the Lighthouse and he was able to figure out that Chronicom's plan was to target and attack all of the active SHIELD bases, although, by the time he realized it, it was too late.