David Marcus

In the fictional universe of Star Trek, David Marcus (2261 - 2285) was the son of James T. Kirk and Dr. Carol Marcus and a major character in the films Star Trek II: The Wrath of Khan and Star Trek III: The Search for Spock. He was played in both films by the late Merritt Butrick. 
David was born in 2261. 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. 
David's professional choices were heavily influenced by his mother's status; by the age of 23 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. 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. 
David's closeted scientific world was interrupted when Khan Noonien Singh, took control of Commander 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. 
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." 
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 naïveté 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, tackling him and briefly taking him to the ground. However, David lacked Kirk's fighting prowess and physical strength; he was quickly overpowered and killed with a single thrust of the three-bladed knife.
David Marcus's untimely death in 2285 was the result of his youthful impatience and fiery temper. He had a brilliant future ahead of him, but he was unable to curb his excitable disposition, and this lack of control cost him his life. He did not die in vain, however, for his actions saved the lives of both Saavik and Spock. Kirk was so enraged over David's death that he lured the Klingons into coming on board the Enterprise just before it self destructed, killing most of the contingent of the Klingon Bird of Prey, and Kirk furthered his vengeance on the planet below, when he shot down the Klingon who had killed David and later sent the Klingon commander falling to his death.