Lieutenant Colonel John Casey, United States Marine Corps [Retired].
Born Alex Coburn, this mean green fighting machine faked his own death so that he could become one of the elitist of the elite. And in doing so, he took on the cover name of John Casey. As John Casey, he eventually joined the NSA and that led him to the Intersect project after he intercepted and seemingly killed Bryce Larkin in the process of stealing it.
As a member of Team Bartowski, Casey acted as Sarah Walker's partner, she being assigned as Chuck Bartowski's handler. That meant his job was to protect and serve both the Intersect and his handler. Eventually, as the government began to assign the team missions, Casey became more of a (tor)mentor to Chuck, trying to show him how to better behave like a spy while reminding him over and over that he's an idiot.
Early on, Casey was shown to be someone who had the NSA's and the US government's best interest at heart. He was even written as the eventual assassin of Chuck whenever the government felt it no longer needed his Intersect, or due to him being a liability. At best, then Major Casey was seen as the man who would put Chuck in a deep dark hole somewhere if the government demanded it.
Things seemed to change toward the end of season 2, as Casey was promoted to colonel (most likely lieutenant colonel, since that is the next rank from major) and tasked with arresting Chuck and Sarah for going against government orders. However, it came to light that Casey was loyal to the team and eventually, especially in season 3, he eased up on Bartowski and became as loyal to him as he had been to the government.
When first assigned to team up with CIA agent Sarah Walker, Casey resisted and even distrusted the blond goddess, but in time the two became true partners, rarely questioning the other, and usually operating as one mind. Later, as Chuck became more of a capable spy, the Casey and Sarah partner dynamic thinned out a bit but they always maintained a loyal friendship and partnership.
As Chuck and Sarah became partners, Casey found himself often partnered with and mentoring the wannabe super spy, Morgan Grimes, who he had worked alongside at the Buy More as a green shirt for several years as his cover job. Morgan was a handful because unlike Chuck, who was brave even though scared to death, Grimes seemed too willing to die. This meant Casey had to reign in him and be even more watchful of the bearded nut. Still, the two developed a strong friendship and eventually moved in together. Not to mention that Morgan would also date Casey's daughter Alex.
Speaking of Alex, she was the daughter he had with his fiancee, Kathleen, who thought he was dead. Casey would tell his daughter the truth when he thought her life was in danger, and eventually confess the same to his ex. Alex has remained a fixture in his life since but Kathleen has not.
Finally, right before the end, Casey reconnected with an old flame named Gertrude Verbanski and they fell deeply in love and lived happily dangerous to the end of their days (which could be any day now).
Was Casey your favorite character? What was your favorite episode with him? Can I stop writing these articles at 9? Or do I really have to do 12 more? Is it sad that I'm doing these from memory?