After Khan and his fellow Augments managed to take control of the USS Reliant, Joachim manned the helm and weapons. When Khan voiced his intention to use the Reliant to take his revenge on James T. Kirk, Joachim attempted to advise Khan against the necessity of taking revenge on Kirk, but deferred to Khan's wishes. Joachim was eventually killed when the Reliant faced the Enterprise in battle in the Mutara Nebula, leading Khan to swear that he would avenge his death. (Star Trek II: The Wrath of Khan)
Joachim was played by an uncredited Judson Scott. Despite his having a prominent role in the film, Scott's name was not listed in the film's credits, thanks to an overzealous agent trying to get Scott star billing without his knowledge.
The vest worn by Joachim in Star Trek II was sold as Lot #821 at the 40 Years of Star Trek: The Collection. The buyer paid US$5,040 for the lot, after winning it with a bid of US$4,200. The catalog, which listed the lot as "JOAQUIM'S VEST", estimated a price of 600 to 800 dollars.
According to Greg Cox's novel To Reign in Hell: The Exile of Khan Noonien Singh, Joachim is the son of Joaquin Weiss and Suzette Ling, who were married after Khan and his fellow Augments were left on Ceti Alpha V in 2267. After Joaquin and Ling both die defending Khan in battle, Khan decides to raise Joachim as if he were his own son. Khan felt this is the least he could do after Joaquin's many years of faithful dedication in service to him. Also noted in the book was the fact that most of the followers of Khan found on Ceti Alpha V by Chekov and Terrell were much younger than Khan. This is explained in the novel that within six months of exile (shortly before the explosion of Ceti Alpha VI), many of the colonists had paired off and several of the women were already pregnant. These children were born in the underground caves following Ceti Alpha VI's destruction and, because of their genetically altered status, they seemed to mature much faster than ordinary humans. For example, when Joachim was ten in approximately 2278, Khan noted in his journals (which were found by Kirk on Ceti Alpha V in 2287) that he appeared already to be fifteen. By the time of their escape aboard the Reliant, all but a handful of the original Augments from the Eugenics Wars era had died due to disease, combat, accident or infection by the Ceti eels. Consequently, most of Khan's crew, which according to Chekov, was only about two dozen people, were the children born after the exile.