- "For the record, I'm always prepared to fire."
- ―Kevin Marks[src]
Kevin Marks is a Major in the United States Air Force and a senior officer aboard several Earth vessels, including the USS Prometheus, the Odyssey, the Apollo, the Daedalus with Marks's most recent assignment being the George Hammond which is under the command of former SG-1 team member, Colonel Samantha Carter.
As a Lieutenant, Marks originally served in an auxiliary control room aboard the Prometheus, where he used the ship's Asgard sensors to detect and map the exterior of a subterranean structure beneath a hilltop in Glastonbury Tor, England. He was able to define a large main cavern as well as a network of smaller tunnels and caves, aiding SG-1's efforts to locate a stash of treasure left there by Merlin. He was also involved in cataloging the items recovered from the site, locating the Ancient long-range communication device among the collection. (SG1: "Avalon, Part 1")
By early 2006, he had been promoted to the rank of Captain. He piloted the Prometheus on its final mission, to destroy an Ori satellite over Tegalus. When the satellite's attacks left the vessel badly crippled, Colonel Lionel Pendergast sent Marks with Lt. Colonel Samantha Carter to attempt repairs to the sublight engines. During their journey through the ship, a second wave from the satellite struck the ship, resulting in Marks's left arm being injured with Carter saving him from falling while Marks was climbing down a ladder. After Marks had his left arm placed in a sling, he and Carter proceeded with their work and although they tried, their efforts ultimately proved unsuccessful. When Pendergast ordered the Prometheus to be evacuated, Carter beamed him into the Caledonian Federation although Marks was clearly hesitant to leave. (SG1: "Ethon")
Following this, he was promoted once again, to Major, and assumed a bridge post aboard the Odyssey, often but not always as the ship's pilot. He took part in the battle with Ba'al's Ha'tak. (SG1: "Off the Grid")
Later, he took part in the Battle of P3Y-229. He was fortunate to survive the massive Ori invasion, as his ship was only one of two out of a dozen ships to survive the battle although the Odyssey suffered great damage after being attacked by the Ori warships. (SG1: "Camelot", "Flesh and Blood")
He joined SG-1 on a mission to a planet that had fallen to the Ori. He detected an energy wave coming from the Stargate and Colonel Paul Emerson ordered him to beam them aboard which Marks did. (SG1: "Counterstrike")
When Lucian Alliance forces under the command of Anateo ambushed the Odyssey and chased it into a minefield, he was knocked unconscious by the attack and captured by the Alliance. He helped Dr. Daniel Jackson and Vala Mal Doran retake the ship and assumed his post on the bridge. When Netan arrived, despite being the ranking officer with Lt. Colonel Samantha Carter trying to repair the hyperdrive, he turned to Daniel to act as the ship's commander and seemed amused by Daniel's efforts to take on that role, correcting him when Daniel told him he "should probably prepare to return fire", telling Daniel he's always prepared, he just has to push a button. When Teal'c contacted the ship asking for a beam-out to the Odyssey, Daniel ordered Marks to do it and Marks beamed Teal'c and Lt. Colonel Cameron Mitchell to the bridge. (SG1: "Company of Thieves")
He and the Odyssey embarked on a mission to an unnamed planet to destroy the Naquadah being stockpiled by Arkad and the Illac Renin. He fired on a Tel'tak that was trying to escape and destroyed it, causing a massive secondary explosion. (SG1: "Family Ties")
Marks piloted the Odyssey on its mission to Orilla. When three Ori warships exited hyperspace, he used the newly installed Asgard plasma beam weapons to destroy one of the ships. Later, he along with the rest of the crew were transported to P3X-474 and used the Stargate to return to Earth after the Ori proved able to track the ship. (SG1: "Unending")
Marks was on the Odyssey with SG-1 on a mission to the Alteran Home Galaxy in order to find the Ark of Truth. He was on the ship to hold it off from being taken over by the rebuilt Replicators. He and Lt. Colonel Samantha Carter barricaded themselves in the Asgard computer core room and searched for the deactivation command. When the Replicators were seen attacking, Marks helped Carter fend off the Replicators with both of them firing P90s at the Replicators with this also marking the first time that Marks was ever seen wielding a gun let alone firing it, suggesting that Marks had recently received weapons training. They were unable to find it in the Asgard computer core but Lt. Colonel Cameron Mitchell was given the location by James Marrick. (Stargate: The Ark of Truth)
Marks was later reassigned to the Apollo when it was upgraded with the Asgard plasma beam weapon. He was taking part in holding off the Asurans and their fleet of Aurora-class battleships so Dr. Rodney McKay and his team could destroy Asuras and wipe out the Pegasus Replicators. This seemed to be more of a temporary posting as he has been seen most recently on board the Daedalus commanded by Colonel Steven Caldwell. (SGA: "Be All My Sins Remember'd")
Marks served on the Daedalus when it went to the planet M2S-445, where Teyla Emmagan was held by Michael Kenmore. He successfully destroyed Michael's cruiser on that mission. (SGA: "Search and Rescue")
It was revealed that he recently gave Teyla training on the Daedalus' systems when the ship was in orbit over Atlantis. He offered it to Ronon Dex as well, but he refused. This training came in handy when the team was stranded on the alternate Daedalus. (SGA: "The Daedalus Variations")
Later on, Marks was in the weapons chair on the Daedalus when Todd arrived and was stunned along with the rest of the bridge crew when the Wraith took over the ship. During the ship's descent towards the surface of M6H-987, he tried to regain control of the ship's systems, finally succeeding moments after Lt. Colonel John Sheppard took the Daedalus and Katana Labrea's ship through the planet in hyperspace. (SGA: "First Contact", "The Lost Tribe")
Marks and the Daedalus later attempted to stop the Super-hive before it was fully upgraded but the Daedalus was utterly outmatched, only managing to escape when the Hive jumped into hyperspace to attack Earth. (SGA: "Enemy at the Gate")
Marks was at the helm of the USS George Hammond during the attack on the Icarus Base in 2009. He detected a massive build-up of energy from the planet. Commander of the 304, Colonel Samantha Carter, ordered Marks to recall Colonel David Telford and his F-302 fighter-interceptors; the ship was badly damaged and they were going to jump to hyperspace. (SGU: "Air, Part 1")
- As the most senior member of the ship's crew after Colonel Paul Emerson's death, Marks could have been expected to take command of the ship, at least while Lt. Colonel Samantha Carter was busy conducting repairs. Instead, he seemed to be taking orders from Dr. Daniel Jackson, a civilian. No explanation was given for this.
- He was promoted twice in the space of one year, from Lieutenant to Captain, then Major. He was a Lieutenant in "Avalon, Part 1" and "Beachhead", Captain in "Ethon", and Major since "Off the Grid".
- Was seen firing a P90 for the first and presumably only time in the movie, "The Ark of Truth".
- To date, he has served on all interstellar vessels produced by the United States, the only ships he has not served on are the Korolev and the Sun Tzu, operated by Russia and China respectively.
- Along with Samantha Carter and Walter Harriman, he is one of only three characters to appear in the final episodes of both Stargate SG-1 ("Unending") and Stargate: Atlantis ("Enemy at the Gate"). All three characters also went on to appear in the Stargate Universe pilot "Air, Part 1".
- Marks is one of only eight characters to appear in all three series, the others being Jack O'Neill, Daniel Jackson, Samantha Carter, Rodney McKay, Richard Woolsey, Bill Lee and Walter Harriman.