David Marcus was a scientist on the Genesis project and the son of James T. Kirk and Carol Marcus, killed by Klingons on the Genesis planet.
David Marcus was the son of James T. Kirk and the scientist Carol Marcus. He was raised by his mother, who kept Kirk out of his life at her request, and grew up to be a scientist working with her on the Genesis project.
In Star Trek II: The Wrath of Khan, David met his father at the Regula I station as Khan Noonien Singh tried to seize the Genesis device. He was suspicious of Kirk and Starfleet at first, but the two made peace before the end of the crisis. In Star Trek III: The Search for Spock, David and Saavik explored the new Genesis planet and found that Spock's body had been reborn. David admitted that he had used unstable protomatter in the Genesis matrix, which was causing the planet to age rapidly and break apart. When Klingons led by Kruge captured them, David was killed trying to protect Saavik and the young Spock.
David's death was one of the heaviest losses Kirk ever suffered and gave the films a lasting note of personal tragedy.