Section 31 is one of Star Trek's most mysterious organizations, and there are only a handful of known of the covert Starfleet Intelligence agency. First introduced in Star Trek: Deep Space Nine, the intelligence organization caught the attention of Dr. Julian Bashir (Alexander Siddig) and Captain Benjamin Sisko (Avery Brooks) during the Dominion War. Section 31 were effectively given carte blanche to use whatever means necessary to protect the Federation, resorting to attempted genocide and intervening in Romulan politics. Section 31 saw themselves above the Prime Directive, and would stop at nothing to achieve their goals.
Despite DS9 implying that Section 31 only surfaced during the Dominion War, Star Trek: Enterprise revealed that the organization had actually been around for almost as long as Starfleet itself. The secretive organization continue to make an impact on Star Trek canon, with a Michelle Yeoh-led Star Trek: Section 31 movie on the way. Section 31 also looks set to resurface in Star Trek's post-Dominion War timeline after adding a Star Trek: Lower Decks character to their extremely secretive ranks. This latest addition to Section 31 is just one of an expanding number of confirmed operatives.
14 Harris (Star Trek: Enterprise)
Harris (Eric Pierpoint) was a former Starfleet Security officer whose official record ends at 2149, when he ed Section 31. Harris is the earliest known member of Section 31, dating the organization to at least the 22nd century. The covert organization to whom Harris answered to was never named on screen, but it was heavily hinted to be Section 31 when he claimed his actions were justified by Article 14, Section 31 of the Starfleet Charter. The audio commentary for the Enterprise episode "Terra Prime" confirmed that it was always the intention that Harris worked for Section 31.
13 Lt. Malcolm Reed (Star Trek: Enterprise)
Lieutenant Malcolm Reed (Dominic Keating) was a former student of Harris, who was recruited into Section 31 prior to his assignment to the Enterprise NX-01. Although Reed left the organization, his former employer later warned him that nobody truly leaves Section 31. Harris used Reed to obstruct the rescue mission mounted by Captain Jonathan Archer (Scott Bakula) to save Dr. Phlox from the Klingons. Reed was left with no choice, and his decision to betray the Enterprise for his former employer put him at odds with his crew mates.
12 Captain Leland (Star Trek: Discovery)
Star Trek: Discovery's Section 31 were widely known within Starfleet, with Leland being an old acquaintance of Captain Christopher Pike (Anson Mount). However, despite Section 31 being a known entity, many of the operations run by Leland were highly classified. Both Project Daedalus and a mission involving the "wrong" ambassador were classified entries in his Starfleet file. Leland was killed by Section 31's sentient threat assessment system which killed him and used his corpse to attack both the USS Discovery and USS Enterprise.
11 Mike And Gabrielle Burnham (Star Trek: Discovery)
The extent of the Burnhams' work for Section 31 appears solely tied to the time travel experiments at Project Daedalus. However, as Harris once told Reed, Section 31 doesn't let its employees go. Gabrielle Burnham (Sonja Sohn) and Mike (Kenric Green) were a husband and wife team of scientists who were researching the possibility of time travel for Starfleet Intelligence. The project was part of a temporal arms race between the Federation and the Klingon Empire, which was overseen by Leland, who blamed himself for Mike and Gabrielle's deaths at the hands of the Klingons on Doctari Alpha.
10 Emperor Philippa Georgiou (Star Trek: Discovery)
A brutal Terran Emperor from Star Trek's Mirror Universe is a perfect fit for Starfleet's off-the-books organization. At some undisclosed point after Star Trek: Discovery's Klingon war, Georgiou was employed by Section 31, and intervened in an attempted coup on the Klingon home world. While working for Section 31, she used her skills to gain valuable intel with which to blackmail her senior officer, Captain Leland, and also recruited Ash Tyler (Shazad Latif) to the organization. Georgiou departed for the 32nd Century with the crew of the Discovery, but was sent back to her own time, where she presumably continued to work for Section 31.
9 Commander Ash Tyer (Star Trek: Discovery)
Commander Ash Tyler was another Starfleet outlier who found a home inside Section 31. A hybrid of the Klingon Voq and Starfleet officer Ash Tyler, he found it hard to fit in and be trusted anywhere, making him an ideal candidate for the secretive organization. After the disaster with Control led to the apparent destruction of the USS Discovery, Ash was placed in command of Section 31, with the hope that Starfleet could reform the organization. It's expected, but unconfirmed that Ash will appear in the Section 31 movie alongside Georgiou.
8 iral Patar (Star Trek: Discovery)
Star Trek: Discovery season 2 revealed the Starfleet irals who oversaw Section 31 in the 23rd century. Among the unnamed Tellarite and Andorian irals, iral Patar (Tara Nicodemo) was the Vulcan iral who ed the use of Control to make key Starfleet decisions. This was a fateful decision, as when Control turned on Starfleet, Patar and her colleagues were the first officers to die.
7 Lt. Kamran Gant (Star Trek: Discovery)
Lt. Kamran Gant (Ali Momen) was another Section 31 operative killed in Control's attack on the Section 31 vessel NI-1101. Gant had previously served alongside Michael Burnham aboard the USS Shenzhou, so Control used his body as a ghoulish means to capture Michael Burnham. Michael was saved by her adopted brother Spock (Ethan Peck) who disabled the nanobots animating Gant's corpse.
6 iral Alexander Marcus (Star Trek Into Darkness)
While Star Trek: TOS had no Section 31, their Kelvin Timeline counterparts had a serious conflict with the organization in Star Trek Into Darkness. The father of Kirk's love interest Lt. Carol Marcus (Alice Eve), iral Alexander Marcus (Peter Weller) was the head of both Starfleet and Section 31. This was a troubling sign of the more militaristic approach that Starfleet was heading following the destruction of Vulcan. In the Kelvin Timeline, the Federation-Klingon War had never happened, but Alexander Marcus knew it was on the horizon. To prepare for this, he took an unconventional approach to strategizing for the future conflict, enlisting the help of a notorious war criminal.
5 Thomas Harewood (Star Trek Into Darkness)
Thomas Harewood was a Section 31 officer who was manipulated by Khan Noonien-Singh (Benedict Cumberbatch) to turn against his colleagues. Khan identified Thomas' sick daughter Lucille as a bargaining chip, and promised to save the child if Harewood did something for Khan. To pay his debt to Khan, Thomas detonated a bomb inside Starfleet Command, which killed 42 officers. The tragedy led to a crisis meeting between iral Alexander Marcus and other key Starfleet personnel, including iral Christopher Pike (Bruce Greenwood) who was assassinated by Khan during his failed attempt on Marcus' life.