Here's a look at every FBI agent who's been assigned to the X-Files division. Understandably, when most people think of The X-Files, they think of agents Fox Mulder (David Duchovny) and Dana Scully (Gillian Anderson). Mulder and Scully were the main stars of nine of The X-Files' 11 seasons, and their chemistry as both colleagues and potential romantic partners was a big reason why the Chris Carter-created sci-fi/horror series became as big as it did. At the same time though, Mulder and Scully were by no means the only agents working on the X-Files during that time.
It's not that Mulder and Scully gave up their jobs in charge of the X-Files division willingly either. The duo often found themselves on the outside looking in thanks to chicanery taking place among their FBI superiors, which itself usually traced back to the machinations of the mysterious Cigarette-Smoking Man and other members of his powerful Syndicate group. Thankfully, Mulder and Scully could usually count on backup from their direct boss, honorable FBI assistant director Walter Skinner.
Still, not even Skinner's interference on their behalf could always shield Mulder and Scully from blowback, including the X-Files being taken away from them. Without any further ado, here's every FBI agent who worked on the X-Files, including both long-term additions and short-lived tenures. However, viewers shouldn't expect to see people like Skinner, Alvin Kersh, or Brad Follmer talked about below, as they were all bosses of the division, not its employees.
Fox Mulder
Before he became, in his words, "the FBI's most unwanted," Fox Mulder was considered one of the best criminal profilers at the bureau. That is until he became obsessed with the X-Files, mostly thanks to memories of his sister Samantha's supposed alien abduction. Mulder investigated X-Files for most of seasons 1-7, a little of 8, and all of 10 and 11. Season 11 ends with the revelation that Scully is pregnant with Mulder's child, but their FBI future is unclear, as Skinner may have been killed.
Dana Scully
Dana Scully was originally assigned to the X-Files to sort of spy on Mulder's work, with the hopes that the skeptical doctor would debunk it. Instead, she grew to like, respect, and eventually love Mulder, and see more than she ever thought she would. Scully spent nearly the entire series assigned to the X-Files, outside of a few periods when the FBI opted to shut down the X-Files division entirely. Like Mulder, her future is uncertain since Skinner may be dead.
John Doggett
John Doggett, played by Robert "The T-1000" Patrick, joined The X-Files in season 8 when Duchovny left as a full-time cast member. In canon, Mulder was abducted by aliens, and Doggett was brought in to be Scully's new partner. He was just as skeptical as she had previously been, if not more so, but they eventually learned to trust each other and became friends as well as colleagues. Doggett would remain a part of The X-Files through season 9, but has oddly vanished since, perhaps due to Doggett's own abduction by the Cigarette-Smoking Man.
Monica Reyes
Monica Reyes (Annabeth Gish) joined The X-Files cast in season 8, and became Doggett's partner in the X-Files division in season 9, when Scully took a step back and decided to teach at the Quantico FBI academy. Reyes and Doggett had a history prior to their X-Files work though, as Reyes had served as the lead FBI investigator when Doggett's son Luke was abducted back when he was a police officer. The two were close friends, and even possible lovers. Sadly, Reyes flipped to the dark side between seasons 9 and 10, going to work for the Cigarette-Smoking Man.
Diana Fowley
When Diana Fowley, played by David Duchovny's The Rapture co-star Mimi Rogers, joined the X-Files unit during season 6, it was an instant source of tension for several reasons. Fowley and Mulder had met during FBI training, and become romantically involved. Fowley was there when Mulder originally discovered the X-Files, and investigated cases with him early on. She and Jeffrey Spender were assigned to the X-Files after Mulder and Scully were booted out following the events of The X-Files' Fight the Future movie. Fowley claimed to be loyal to Mulder, but was also secretly working for CSM and the Syndicate. She eventually chooses to help Mulder escape a Syndicate predicament, leading to her murder.
Jeffrey Spender
As mentioned, Jeffrey Spender (Chris Owens) was assigned to work on the X-Files alongside Diana Fowley. Spender is initially antagonistic towards Mulder, making the reveal that he's actually the son of the Cigarette-Smoking Man - aka Carl Gerhard Busch Spender - less surprising that it perhaps should've been. Spender is hardly a fan of his father's though, blaming him for the traumatized mental state of his mother Cassandra. He eventually turns on his dad and gets CSM's other son Mulder and Scully reinstated to the X-Files. He gets shot by CSM in retaliation, but in season 9 is revealed to have survived, only to be horribly experimented on. He returns again, looking much better, in the revival.
Alex Krycek
A brief but insidious addition to the X-Files division during season 2 was Alex Krycek, who was assigned to replace Scully after her abduction and disappearance. As any X-Files fan now knows, Krycek was a mole for the Cigarette-Smoking Man, and would fairly quickly turn on Mulder. Krycek became a fan-favorite recurring villain from then on, before finally being shot and killed by Skinner during season 8. Krycek did occasionally help Mulder and Scully out over the years though, and his ghost even appeared to Mulder during season 9, as part of many returns during The X-Files' original series finale.
Leyla Harrison
The story behind the creation of brief X-Files agent Leyla Harrison is both heartwarming and sad. Harrison was named after an X-Files super-fan who wrote X-Files fanfiction and passed away due to cancer in 2001. In the earlier days of internet fandoms, The X-Files was one of the earliest shows to develop a strong online following, and their devotion was appreciated. Harrison is essentially a tribute to X-Files fans, as evidenced by her encyclopedic knowledge of past X-Files cases. She works alongside Doggett in season 8's "Alone" and season 9's "Scary Monsters," and thankfully doesn't get killed along the way.
Kyd Miller & Liz Einstein
One of the best inventions of the X-Files' two-season revival was the introduction of agents Kyd Miller (Robbie Amell) and Liz Einstein (Lauren Ambrose), two young agents who work with Mulder and Scully on X-Files cases a few times. Miller and Einstein essentially mirror Mulder and Scully's original dynamic, with Miller open to the paranormal and supernatural, while Einstein is highly skeptical of such things. The strongest moments see Mulder working with Einstein, and Miller working with Scully, as their interplay is great. Both Miller and Einstein survived the revival, and some consider it a disappointment that they didn't get a spinoff.
Arthur Dales
A mention of FBI agents who've worked on the X-Files wouldn't be complete without Arthur Dales, one of the earliest people to take on the task of investigating the unexplained. Unfortunately, this path didn't work out for him, as he toiled at the X-Files for a while, then quietly retired without much fanfare. Dales had previously worked alongside Mulder's father Bill, and was later sought out for assistance with a case by Mulder. He would later ask for Mulder and Scully's help as well. Dales was played by Darren McGavin, who starred in the classic 1970s series Kolchak: The Night Stalker, from which X-Files creator Chris Carter drew inspiration when formulating his own show.