David Marcus was a male Human scientist in the 23rd century United Federation of Planets. Born in 2261 to James T. Kirk and Dr. Carol Marcus, he grew up in a single-parent environment with his mother, immersed in a world of scientific research. David knew that Kirk was once part of his mother's life, but the boy had no idea that the famous Starfleet admiral was his father, nor that his mother effectively banned Kirk from his life; she felt they lived in two different worlds, and she wanted David in hers.
David's professional choices were heavily influenced by his mother's status; by the age of twenty-three, he had earned his doctorate and soon became a member of his mother's research team.
In 2284, David began work on Project Genesis with his mother, stationed on Regula I. As they progressed, traces of his father showed in David; he was impatient for results and eager for solutions. This led him to employ protomatter, denounced by many scientists as hazardous and highly unstable. Nevertheless, it appeared to solve the problems.
In 2285, David's closeted scientific world was interrupted when Khan Noonien Singh took control of Pavel Chekov and forced him to announce that he was taking Genesis away from Carol's team on orders from Kirk. This threat was part of Khan's ruse to lure Kirk to them, and it provided the volatile backdrop to the first meeting between father and son. He fled with his mother to the Genesis cave to hide their research from Starfleet. Khan could not find them, but Kirk arrived afterwards and knew to look for them there, beaming down.
This encounter actually began as more of a confrontation; David displayed the same eagerness for action as his father when he attacked the admiral with a knife, presuming that Kirk's intent was to take Genesis. Carol intervened, and it was after seeing her that Kirk guessed David was his son. David got a chance to see his father in a more positive light when Kirk battled it out with Khan. David remained on the bridge of the USS Enterprise, acting as advisor on the Genesis wave, and was impressed by his father's courage and abilities.
After this brush with death, David showed his compassion by trying to comfort the man he once despised, when Kirk was grieving over the death of Spock. David apologized to Kirk for misjudging him and added, "I'm proud, very proud, to be your son." (Star Trek II: The Wrath of Khan)
Shortly afterward, David was assigned to the USS Grissom with Lt. Saavik to study the planet created in the Genesis explosion. At first, he was thrilled with the array of environments, but when he beamed down for a closer look, he found the planet in a state of environmental chaos, and recognized the cause as being the protomatter's instability.
David's naivete showed through when a group of Klingons arrived and demanded the Genesis Device. He honestly believed the Klingons wouldn't harm them because, as a planet-building device, it was an obvious failure. To the Klingons, however, Genesis was a powerful weapon, and a hostage was selected to die as proof of their serious intent.
As a d'k tahg knife was raised to Saavik's neck, David lunged toward the Klingon in a burst of courage which might have been expected of his father. However, he lacked Kirk's fighting prowess, and was killed with a single thrust of the three-bladed knife to his chest. (Star Trek III: The Search for Spock)
The memory of David's death at the hands of the Klingons embittered Kirk for years afterward and colored his attitudes during events leading to the Khitomer Conference in 2293. At that point, Kirk proclaimed the Klingons were untrustworthy and favored simply allowing them to die, due to his old grief and rage over their role in the death of his son. (Star Trek VI: The Undiscovered Country)
"Every time we have dealings with Starfleet, I get nervous. We are dealing with something that... could be perverted into a dreadful weapon. Remember that overgrown Boy Scout you used to hang around with? That's exactly the kind of man..."
"Listen, kiddo, Jim Kirk was many things, but he was never a Boy Scout!"
- - David and Carol (Star Trek II: The Wrath of Khan)
"I've tried to tell you before, scientists have always been pawns of the military!"
- - David to Carol (Star Trek II: The Wrath of Khan)
"Where's Dr. Marcus?"
"I'm Dr. Marcus!"
- - Kirk and David's first exchange, before he realizes he is his son. (Star Trek II: The Wrath of Khan)
"You knew enough to tell Saavik that how we face death is as least as important as how we face life."
"But good words. That's where ideas begin. Maybe you should listen to them. I was wrong about you... and I'm sorry."
"Is that what you came here to say?"
"Mainly. And also that I'm... proud, very proud... to be your son."
- - David and Kirk (Star Trek II: The Wrath of Khan)
"This is where the fun begins, Saavik."
"Just like your father – so Human."
- - David and Saavik when the Grissom arrives at the Genesis Planet (Star Trek III: The Search for Spock)
"Hello, sir. It's David."
"I'm sorry I'm late."
"It's OK; I should've known you'd come. But Saavik's right. This planet is unstable; it's going to destroy itself in a matter of hours."
"David, what went wrong?
"I went wrong."
"I don't understand."
"I'm sorry, sir. Just don't surrender. Genesis doesn't work. I can't believe they'd kill us for it."
- - David and Kirk (Star Trek III: The Search for Spock)
David Marcus was played by Merritt Butrick. When casting Butrick in the part, director Nicholas Meyer found the actor's appearance to be acceptable. "He not only had to be Kirk's son, he had to be Carol's son," Meyer recalled, "so on a physical level I think what I liked was that his hair was the same color as hers but it was curly like Bill's, so I thought, 'Well, that's plausible.'" (Star Trek: The Magazine Volume 3, Issue 5; ) Butrick himself liked that the storyline of Star Trek II focused on the characters but he also felt somewhat "odd" about playing David because doing so involved interacting with voices he was highly familiar with, having been "raised on Star Trek." (The Making of the Trek Films, 3rd ed., p. 171)
According to the novelization of Star Trek III: The Search for Spock by Vonda N. McIntyre, Lt. Saavik is supposed to have had a short relationship with David Marcus. The Mirror Universe Saga establishes that, in the mirror universe, David Marcus was believed killed by Kirk, but he had survived and was leading the rebellion against the Terran Empire on Earth.
In the mirror universe story The Sorrows of Empire (which takes place in a different continuity from The Mirror Universe Saga), Carol Marcus thanks Emperor Spock for killing Kirk as she had lived in fear for years of what he would have done had he known about David's existence. Spock tells Carol that she was right to fear Kirk, as he would have killed them both.
The Star Trek: Myriad Universes novella "A Less Perfect Union" – set in an alternate quantum reality where Earth cut off contact with the rest of the galaxy due to the success of Terra Prime in 2155 – establishes that David, as well as his mother Carol, (who in this universe, had married Kirk), had been killed aboard an UESPA starship, en route to a conference on Vulcan, their ship being destroyed by a Vulcan ship near the United Earth/Interstellar Coalition border, which embittered Kirk for years toward Vulcans.
Additionally, David is a main character in "The Chimes at Midnight", set in the alternate timeline from TAS: "Yesteryear" where Spock died as a child in 2237. In this timeline, because there is no Spock for Kirk to come rescue, David and Saavik are taken aboard Kruge's Bird-of-Prey and David is subsequently tortured for information about Genesis before he and Saavik are rescued by the Enterprise crew, although Kirk sacrifices his life and destroys the Enterprise to save everyone else. David and Saavik cultivate a relationship and, as relations between the Romulan Star Empire and the Federation deteriorate, David remains on Romulus to live with Saavik.