Lieutenant Valeris was a Starfleet officer who served as helm officer 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.
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.