As a Chameloid, she was able to change her appearance at will. One seemingly preferred guise was as a female humanoid dressed in a dark brown fur outfit (complete with scarf and including a jacket or coat), smoking a cigar. Other appearances included a small child or a large male alien.
In 2293, she was involved in a plot to kill Captain James T. Kirk and Doctor Leonard McCoy. Martia was offered a full pardon for her crimes if she assisted the Klingons in murdering the two Starfleet officers, by helping them escape and therefore giving the guards a believable excuse for killing them. She initially approached Kirk and McCoy when they were imprisoned in Rura Penthe, and helped them avoid a fight with a large, threatening inmate as well as with a horned alien prisoner. Then, she offered them a way to escape.
Using her shapeshifting abilities, she alternately appeared as a hulking, hirsute male alien to infiltrate a male-only level, and then as a little blond girl to escape her restraints. Helping Kirk and McCoy travel beyond the magnetic shield, Martia then attempted to alert the guards to their location, and engaged in a fight with Kirk, changing her appearance again to mimic Kirk's features. When the commandant of the colony arrived, however, he was tricked into killing Martia by the genuine Kirk. (Star Trek VI: The Undiscovered Country)
Appearances of Martia
Martia, appearing as she primarily did, was played by Iman. In the script for Star Trek VI, this appearance was described thus: "She's a tall and very good looking female, boots to her thighs, exotic furs, tattered but clearly humanoid." 
Screenplay co-writer Denny Martin Flinn initially considered Sigourney Weaver for the part. "The person I had in mind," he said, "was as different as night and day from Iman [....] I'm not sure we didn't come up with a better choice. I didn't imagine that we could ever afford [...] [Weaver], but I just saw the character as a big, ballsy space pirate; a female version of Han Solo," Flinn explained. "She turns out to be rather villainous, but a kind of swashbuckling female space pirate, and I thought of Sigourney." According to producer Ralph Winter, the reasoning behind the casting of Iman was not only because she was thought to be a high-caliber performer but also because she was considered to have an appropriate look. (The Making of the Trek Films, UK 3rd ed., p. 108)
Iman was more-or-less cajoled into accepting the role. She remembered, "I was in Venice, Italy, when I was offered the part. And I kept on thinking, 'I don't know. I mean, it's an alien. Do I want to play an alien?' And [her manager] is such a fan that he told me, he gave me no opportunity. He said, 'You're going to do it and that's it. We're not even going to discuss it.'" (Original interviews: Iman, Star Trek VI: The Undiscovered Country (Blu-ray) special features)
Iman used her own accent to play Martia. Her voice was ultimately looped in the studio for most of the character's other appearances. (Original interviews: Iman, Star Trek VI: The Undiscovered Country (Blu-ray)special features)
In an audio commentary for Star Trek VI (available on the film's special edition DVD release), director Nicholas Meyer tells Denny Martin Flinn that Iman did not wear contact lenses for this role. However, Iman actually did don contact lenses, noticeably so. "They're a pain," she laughed. "It's very uncomfortable. They're soft lenses. They're amber color with sparkles of black to make it look very bird-like [....] It's very uncomfortable to wear them for a long period of time. To put them in, to put them out, they're uncomfortable. So they also mess with the makeup, 'cause you have to retouch everything every time when you finish makeup and they put them in. So it's really a pain." (Original interviews: Iman, Star Trek VI: The Undiscovered Country (Blu-ray) special features)
"I think, from the outfit, they couldn't decide whether it was a bird or cat," stated Iman, with a laugh, "so they came up with something in between." (Original interviews: Iman, Star Trek VI: The Undiscovered Country (Blu-ray) special features) Her alien appearance (what Ed French termed, "Vogue in space," in recognition of Iman's modeling work for Vogue) was designed after Nick Meyer described the character as "Kirk's fantasy woman." To preserve Iman's attractive features, French chose to give her a simple makeup design. He and hairstylist Janice R. Alexander devised a feathered look. "Some people may think a lot of what's on her head is hair," French suspected, "but everything was made of feathers. Even her eyebrows were little tiny feathers which we applied. We also attached feathers to the sides of her face and neck." (Cinefantastique, Vol. 22, No. 5, pp. 33-34) "All the crew and the cast are enamored by my look now. So, I don't know, maybe I'll keep it," Iman joked. (Original interviews: Iman, Star Trek VI: The Undiscovered Country (Blu-ray) special features)
Iman was conscious of paying attention to details about the other performers playing Martia's range of guises and then tried to mimic their habits. (Original interviews: Iman, Star Trek VI: The Undiscovered Country (Blu-ray) special features)
Martia, appearing as "the Brute", was played by Tom Morga. His makeup for the part was inspired by a sketch of the creature that Star Trek VI art director Nilo Rodis-Jamero illustrated. An unrefined version of the Brute's makeup was speedily created over a period of three weeks, for location filming in Alaska. "Fortunately, for those shots, it was only supposed to be seen from some distance," noted Ed French. "I wasn't able to refine the makeup until it got back to the studio." Costumer Dodie Shepard fabricated the Brute's bodysuit, while French was responsible for the creature's hands and feet, the latter crafted to fit over "huge Frankenstein boots" that Shepard provided. (Cinefantastique, Vol. 22, No. 5, p. 33) The very heavy makeup required for the part took about three and a half hours to apply. (The Making of the Trek Films, UK 3rd ed., p. 110) Morga remembered of the Brute, "He looked like the abominable snowman [....] I had to learn to walk and move with him, because [I] wore so much stuff. I had to have these great big boots on, and the suit they built was an arctic suit to start with. So, it's got down in it. I was very warm, and over that they put layers of animal pelts for this character. And I was all in rubber that was glued to me, all my face, my arms. And I had this huge wig on, so I was really hot. Just walking over the ice and up and down the glacier itself was a little bit of a challenge." ("Tom Morga: Alien Stuntman", Star Trek VI: The Undiscovered Country (Special Edition) DVD)
In her appearance as a small Human girl, Martia was played by Katie Jane Johnston. This girl was scripted to appear "teenaged". According to Iman, however, she was only nine years old. (Original interviews: Iman, Star Trek VI: The Undiscovered Country (Blu-ray) special features)
In her guise as a duplicate of James T. Kirk, Martia was played mostly by an actor who was dressed identically to William Shatner but whose face is never clearly shown and by Shatner himself in a few motion-control split-screen shots. (text commentary, Star Trek VI: The Undiscovered Country (Special Edition) DVD) Although Iman's voice was used for all of Martia's previous appearances, this was not so for her Kirk form. At one point years later, Nick Meyer regretted that the production team had not consistently used Iman's voice for all of Martia's appearances (reckoning that it would have made for a funnier scene), though he then realized that this would have instantly made Martia, while disguised as Kirk, recognizable from the real Kirk. (audio commentary, Star Trek VI: The Undiscovered Country (Special Edition) DVD)
Although the movie does not clarify the extent of Martia's criminal behavior prior to her detention in Rura Penthe, a draft version of the Arc.contains additional dialogue between Kirk and Martia in which she states that she is serving a life sentence for smuggling, which she admits to having been guilty of. The script also specified her to be from a place called
The use of CGI to depict Martia's transformations was one of the first instances when morphing was used in feature films, though CGI had previously been utilized for television commercials. (audio commentary, Star Trek VI: The Undiscovered Country (Special Edition) DVD) The CG effect was visualized by Industrial Light & Magic, which had developed the morphing technology for Willow and had reused it in Terminator 2: Judgment Day. Jay Riddle, who supervised ILM's morphing between Martia's various appearances, explained, "This time, computer graphics animator John Berton attempted to do some things we hadn't done before in a morph–like moving the camera and letting characters talk as they were transforming–so a lot of care had to be taken in the plate photography to make sure the two characters lined up." The filming of the on-set plate photography was overseen by Scott Farrar. Upon filming the sequence wherein Martia transforms from the Brute to the small girl, Farrar utilized ILM's VistaGlide motion-control camera system to incorporate camera moves into the sequence. For Martia's mid-sentence transformation between her usual appearance and that of Kirk, Farrar had performers Iman and William Shatner sync their deliveries to a video playback whose audio was relayed over loudspeakers. (Cinefantastique, Vol. 22, No. 5, p. 50)