Lieutenant Valeris was a female Vulcan Starfleet officer of the 23rd century. She served as helmsman aboard the USS Enterprise-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.
Valeris was mentored by Captain Spock at Starfleet Academy, and became the first Vulcan to graduate at the top of her class. Spock took interest in her career with "satisfaction" as she eventually attained the rank of lieutenant.
Valeris requested to be assigned to the Enterprise-A upon learning of their need for a helmsman. Spock, having been impressed with her, informed her of his intentions to retire subsequent to the completion of their diplomatic mission, and of his desire that she should fulfill his role aboard the ship.
Valeris did not consider it logical to attempt peaceful interactions with the Klingon Empire, considering the Klingons untrustworthy. Before coming to the Enterprise, Valeris had committed to the Khitomer conspiracy on the Starfleet side. Upon establishment of contact with the chancellor's flagship, Kronos One, 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.
Spock ordered Valeris to conduct a search for evidence of the conspirators on the ship. She helped to uncover a few facts and the boots that the assassins wore. Covertly, she also personally murdered Yeomen 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, leading to a confrontation with Valeris when she made another assassination attempt.
She was taken to the bridge and interrogated by Kirk in front of the crew for details of the conspiracy. Valeris refused to cooperate, instead appealing to Kirk's distrust of Klingons. In response, Spock forced a mind meld on her, during which he was able to learn of the involvement in the conspiracy of Starfleet Admiral Cartwright, Klingon General Chang, and Romulan Ambassador Nanclus. She was not aware of where the Khitomer Conference was to take place, but did know of another assassination attempt that was to occur there. Hikaru Sulu instead informed them, and joined with them at Khitomer. There, Valeris was taken to the conference to show everyone they could prove the conspiracy. (Star Trek VI: The Undiscovered Country)
Saavik was to have reprised her role in Star Trek VI: The Undiscovered Country, 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. In addition, no one wanted to have to cast a third actress to play the same role: Kirstie Alley had declined to return, and Nicholas Meyer was not a fan of Robin Curtis's portrayal.
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, contradicting, however, the typical naming convention for female Vulcans (T'Pring, T'Pol, and T'Pau, for example). (I Am Spock) The shooting script spells the name "Val'eris", but the apostrophe doesn't appear in the closing credits, DVD subtitles or publicity materials.
Jeanne M. Dillard's novelization of Star Trek VI: The Undiscovered Country mentions that her parents Sessl and T'Paal attempted to make peace overtures to the Klingons several months before the Organian intervention. Her mother, who named Valeris for a Klingon heroine, is killed and her father becomes ill. Sessl dies when Valeris is seven, advocating that war with the Klingons is necessary.
The novel Cast No Shadow depicts Valeris' life both before and after Star Trek VI: The Undiscovered Country, when she is offered a chance at relocation to a colony world under a new name, seven years after her arrest and imprisonment, in exchange for assisting in an investigation into what appears to be a new branch of the Khitomer Conspiracy as the last known living member of the scheme. During the events of the novel, she is forced to acknowledge that the real reason for her participation in the conspiracy was not her logical assessment that the Klingons would be a future threat to the Federation, but her own past trauma from a time she was taken hostage as a child by Klingons attempting to convince her ambassador father to abandon a planet that was negotiating to join the Federation. At the conclusion of the story, she risks her life to disable a bomb that would have decimated the remains of Praxis and the Klingon homeworld, and is given a pardon and a new life on a colony world under the new name T'Leris.