Maltz in 2285.
Maltz was a Klingon officer who served on the Bird-of-Prey commanded by Kruge during the 2280s.
In 2285 he was present during the mission to the Genesis Planet to discover its secrets. Maltz was the only member of the crew not to join the landing party to the planet below, nor the boarding party to the doomed USS Enterprise and, accordingly, was the only member of the crew to survive the mission. Maltz was taken prisoner when the Bird-of-Prey was commandeered by Admiral James Kirk and his crew. Kirk said he would kill Maltz later but later said he lied. (Star Trek III: The Search for Spock)
Background
Maltz was played by actor John Larroquette. It is unclear what became of him after Kirk took the Bird-of-Prey to Vulcan. He may have become a Federation prisoner of war, though the Klingon Ambassador makes no mention of him when demanding Kirk's extradition in Star Trek IV: The Voyage Home.
Marc Okrand in his books describing the Klingon language suggested that Maltz may have revealed the Klingon language to the Federation and to Marc Okrand. This, of course, contadicts what has been seen on Star Trek: Enterprise, in which Vulcans had the language stored in their database from previous contacts and Hoshi Sato was able to translate the language.
Aprcrypha
In the novelization of the movie, it is stated that he committed suicide. In the Genesis Wave series, however, it was revealed that Maltz survived, but was disgraced by his loss of honor, he hid away from Klingon society for several years before the coming of the Genesis Wave. With the Wave, Maltz's life gained a new purpose, and he tracked the wave to its source, before, accompanied by Carol Marcus, he set the station to self destruct, regaining his lost honor.