Valeris

Lieutenant Valeris was a Starfleet officer who served as helm officer aboard the USS Enterprise (NCC-1701-A), during its mission to escort the Klingon Chancellor to Earth in 2293. In that capacity, she was a key participant in a conspiracy to sabotage peace talks between the Klingon Empire and the Federation.

Prior to her assignment to the Enterprise, Valeris was mentored by Captain Spock at Starfleet Academy, and became the first Vulcan to graduate at the top of her class. So impressed was Spock that, upon being reunited with her aboard the Enterprise, he informed her of his intentions to retire subsequent to the completion of their mission, and of his desire that she should fulfill his role aboard the ship.

Upon establishment of contact with the chancellor's flagship, Valeris suggested to Captain Kirk the service of Romulan ale during a dinner which the Enterprise crew was hosting for Chancellor Gorkon and his party. Later, after Kirk and Doctor McCoy were placed under arrest for Gorkon's murder, Valeris supplied her compatriots in the Klingon Empire with a recording from Kirk's personal log, in which Kirk admitted that he held the Klingons personally responsible for the death of his son, which became a damning piece of evidence in their subsequent trial. Valeris also personally murdered Yeomans Burke and Samno, her fellow conspirators and Gorkon's actual assassins, in an attempt to eliminate them as witnesses.

After the Enterprise crew's successful rescue of Kirk and McCoy from the Klingon prison planet Rura Penthe, Kirk and Spock set up a sting operation in an attempt to uncover the conspirator aboard the ship, by announcing that Burke and Samno had, in fact, survived the murder attempt, and were recuperating in sickbay. Their ruse proved successful, and they confronted Valeris as she entered sickbay in another assassination attempt. Captain Kirk subsequently attempted to interrogate Valeris as to the details of the conspiracy, but was met with frustration as Valeris refused to cooperate. In response, Spock forced a mind meld on Valeris, during which he was able to learn of the involvement in the conspiracy of Starfleet Admiral Cartwright, Klingon General Chang, and Romulan Ambassador Nanclus, as well as the fact that Captain Sulu would be able to supply them with the location of the imminent Khitomer Conference, wherein another assassination attempt would take place. 

Background
Saavik was to have reprised her role in, but was replaced with Valeris, reportedly because Gene Roddenberry was concerned that fans had become so fond of Saavik that they would react negatively to her turning out to be a traitor. Also, no one wanted to have to cast a third actress to play the same role.

While referred to as "lieutenant", Valeris wore the rank insignia of a lieutenant commander. Her Starfleet uniform jacket had the gray insignia patch and wristband of a sciences division officer, but her uniform undershirt was red (denoting a cadet or student officer). This mismatch was caused because no new uniform pieces were being made and the wardrobe department simply found Cattrall existing items that fit her. Robert Fletcher's uniform notes specify that an officer's collar and uniform patch should match, so a division specialty color could have been added as a strip on her shoulder patch and wristband.''

Kim Cattrall designed her headband, suggested her hairstyle and even helped in naming the character – after Eris, the Greek goddess of strife, hinting at Valeris' role in the events of the film. The "Val" prefix was added to make it sound more Vulcan. (I Am Spock) The shooting script spells the name "Val'eris", but the apostrophe doesn't appear in the closing credits or publicity materials.

Cattrall studied the life of Che Guevara when she prepared for playing her role, according to an article by Mark A. Altman.

The novel Cast No Shadow depicts Valeris' life both before and after Star Trek VI: The Undiscovered Country. The mirror universe Valeris appeared in the novel The Sorrows of Empire.