David Marcus

Doctor David Marcus was the son of Carol Marcus and James T. Kirk. Marcus did not learn that he was Kirk's son until 2285, shortly before his death. 

History
In the year 2268, approximately halfway through the USS Enterprise (NCC-1701)'s five-year mission (on stardate 3998.6), the seven-year-old Marcus and his mother were involved in a Klingon attack upon the Federation colony world of Beta Canzandia, where a terraforming research project was underway, supervised by Carol Marcus. Escaping into the hills with Commander Spock and the rest of the colony's children, David survived until the group could be rescued by the Enterprise, but would harbor deep distrust and resentment against Starfleet for many years following the incident. 

When he was sixteen years old, Marcus vacationed at the Sojourner Ranch on Mars, where he was injured and nearly bled to death. He attended the Daystrom Institute, where he once attended a guest lecture by Spock, and graduated at the age of twenty-two. (SS TOS (Prodigal Father))

He was one of the leading scientists behind the creation of the Genesis Device. Marcus expressed misgivings in dealing with Starfleet, knowing that Genesis could be used as a terrible weapon. When Khan Noonien Singh had the USS Reliant (NCC-1864) come to Regula to take the Project Genesis device and related materials, Marcus and the other Genesis Project team members did not know that Khan had hijacked Reliant. David Marcus believed that Starfleet was going to co-opt Genesis for military use; however, his mother did not share that view.

Things only got worse for them when the USS Enterprise (NCC-1701), on a training cruise and ambushed by the Reliant, arrived at Regula One space station to get answers. When Admiral James T. Kirk, who was put back in command of the Enterprise due to the sort of military crisis, arrived in the Genesis Cave, Marcus attacked Kirk, believing him to be responsible for the murder of the scientists who remained behind on the Regula I station. Captain Clark Terrell and Commander Pavel Chekov - under the control of Khan via Ceti eels - used the distraction to pull weapons on the Enterprise crew members. They provided the coordinates of the Genesis torpedo to Khan. David Marcus tried to rush Terrell, and Lieutenant Saavik tackled him before Terrell could vaporize him with a phaser. Terrell and Chekov were unable to bring themselves to kill Admiral Kirk. Terrell committed suicide, and the eel left the body of Chekov, who later recovered. 

During the final battle between the Enterprise and Reliant, Marcus went to the bridge. When an overloaded helm console severely injured Hikaru Sulu, Marcus performed CPR on Sulu until medics could arrive on the bridge, saving Sulu's life. When a defeated Khan activated the Genesis Device, Marcus recognized the Genesis wave emanating from the Reliant. Marcus told Kirk that they had four minutes before the device exploded, and that once started it could not be stopped. 

During the battle, Marcus came to feel immense pride in being Kirk's son. (SS TOS (Prodigal Father))

A short time later David Marcus and Saavik transferred to the USS Grissom (NCC-638), which was heading back to the Genesis Planet to conduct a science survey. During this time the two had a relationship. Arriving at the planet, the two discovered an animal life form on the planet which should not have been there. The two beamed down to Genesis to investigate, and discovered the regenerated body of Captain Spock. Right after discovering Spock on the surface of Genesis, the Grissom was destroyed by a Klingon bird of prey under the command of Kruge, who was attempting to steal the secrets of the Genesis Device. Seeing that both Spock and the planet were rapidly aging, Marcus admitted to Saavik that he had used protomatter to create Genesis.

Marcus, Saavik, and Spock were taken prisoner by the Klingons, and Marcus was killed at Kruge's order. 

David's death deeply affected his father, and caused him to harbor great distrust and anger toward the Klingons for years to follow. 

While trapped in the Nexus, Captain James T. Kirk experienced visions of David and Carol Marcus. 

Alternate timelines
In an alternate timeline in which John Frederick Paxton destroyed Starfleet Command and ended the talks for the Coalition of Planets in 2155, David Samuel Kirk was born to Commander James T. Kirk and his wife Carol Kirk, who married in 2255.

In 2264, the three-year-old David and his mother were killed when their transport vessel, the Galileo, was travelling to an interstellar symposium on molecular biology proposed by the Vulcan government. Upon the sight of a human vessel crossing the border of the Interstellar Coalition, the ship was destroyed by the Coalition vessel Vanik when its Vulcan captain T'Prynn claimed that the human transport was not responding to hails. This led to his father developing a deep resentment towards Vulcans. 

In another alternate timeline where Spock died in childhood in 2237, David and Saavik were investigating the newly formed Genesis Planet when they were taken prisoner by Kruge. Having heard about the power of the Genesis Device, Kruge and his crew tortured David for information going so far as to sever his left hand. Nevertheless, David refused to divulge any information, and, with Saavik's help, was able to bury any knowledge of Genesis from being detected by the mind-sifter device.

David and Saavik were eventually taken to Praxis for a period of about one month before Kruge rendezvoused with Kirk and the recently stolen USS Enterprise (NCC-1701) to exchange the two prisoners for information on Genesis. Although David survived, Kirk died while trying to save the Enterprise from a warp core breach.

Later, when war broke out with the Klingon Empire over the Federation's refusal to share the technology of the Genesis Device, David worked as a social worker, recording the names of Federation refugees arriving on Vulcan. David and Saavik were romantically involved at this time, but saw little of each other as Saavik was acting as the Federation ambassador to the Romulan Star Empire and thus spent most of her time on Romulus.

As the war dragged on to a stalemate, David was asked by the Federation President Ra-ghoratreii to be involved in discussions on the use of the Genesis Device on the Klingon moon of Praxis. Known as Operation Olympius, David refused and would have no part in it.

Following the Federation's attack on Praxis and the Klingon Empire's subsequent surrender, David briefly worked on Qo'noS as part of the Federation's medical relief effort before deciding to join Saavik on Romulus.