This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
Ash Tyler, formerly known as Voq, was a 23rd century Klingon warrior who underwent the choH'a', an extreme, agonizing surgery so that he would appear to be Human during the Federation-Klingon War. His DNA and memories were somehow spliced with captured Starfleet officer Lieutenant Ash Tyler in the experimental procedure, making him an artificial hybrid. His mission was to become a sleeper agent, and by the time the process was complete, Tyler's personality had completely subsumed Voq's.
As Ash Tyler, he became the chief of security of the USS Discovery, where he began a relationship with Michael Burnham. Voq's memories began to resurface after his compatriot L'Rell was captured by the Discovery, but the Ash Tyler persona's attraction to Burnham prevented them from being restored completely. However, after the Discovery was transported to the mirror universe, Voq met his counterpart, which caused his full Klingon personality to be restored. Voq unsuccessfully attempted to murder Burnham before being detained by the Discovery crew. Later, surgery caused the two personalities to merge together, leaving the surviving personality, that of Tyler, guilt-ridden and ashamed. Ultimately, when L'Rell left to take leadership of the Klingon High Council, Tyler chose to leave the Discovery to aid both the Federation and the Klingon Empire, having decided he was no good for either side alone. (DIS: "Will You Take My Hand?")
Under the guise of Ash Tyler, Voq believed he was held captive for 227 days, almost seven months. His cellmate during his captivity was Harcourt Fenton Mudd. During his time with Mudd, he listened to his story of how he robbed a bank on Betazed. He was "rescued" along with Captain Gabriel Lorca by the USS Discovery, all according to L'Rell's plans. (DIS: "Choose Your Pain", "Magic to Make the Sanest Man Go Mad")
A week after this, Captain Lorca appointed "Tyler" as his new chief of security aboard the Discovery. Captain Lorca was impressed by his skill piloting a stolen Klingon raider during their escape, as well as his impressive marksmanship in Discovery's holographic combat simulator.(DIS: "Lethe")
"Tyler" appeared to have suffered from severe post-traumatic stress disorder as a consequence of rape, torture, and medical experimentation inflicted upon him during his imprisonment by his Klingon "captor", L'Rell. (DIS: "Into the Forest I Go") Furthermore, he harbored an internal rage towards Klingons, wanting to hurt them the way they hurt him. (DIS: "Si Vis Pacem, Para Bellum") During his mission with Burnham to place sensors in the Klingon Ship of the Dead, "Ash" encountered L'Rell in a room of corpses, and subsequently lapsed into catatonia. He was coached back to a conscious state by an injured Admiral Katrina Cornwell, who was a prisoner on the ship, and was able to defeat incoming Klingon warriors who were closing on their position. L'Rell was subsequently taken prisoner and transferred to Discovery's brig. (DIS: "Into the Forest I Go")
"Tyler" was still haunted by nightmares and flashbacks from his violations, and what was later revealed to be his surgical transformation from Voq to the Human form of Tyler. He took solace in his burgeoning romantic relationship with Burnham, who had shared trauma from the loss of her parents, her transient death, and the events from the Battle of the Binary Stars.
When the Discovery found itself in the mirror universe, "Tyler", who was experiencing nightmares of surgery being performed on him, went to confront L'Rell in the brig. L'Rell recited a Klingon prayer to him, which seemed to trigger a reaction. Disturbed by this, "Tyler" asked Doctor Hugh Culber to perform a medical examination on him to see if he was fit for duty. Culber discovered that, contrary to earlier examinations, his patient's skeletal structure and organs had been surgically altered and that his personality had been overlaid the personality of someone else. Culber said he had to ground "Tyler" because he was not sure that he was who he claimed to be. In response, "Voq" became self-aware and broke Culber's neck before accompanying Burnham and Lorca on a mission to the ISS Shenzhou. (DIS: "Despite Yourself")
While aboard the Shenzhou, Tyler and Burnham were able to gain the trust of a rebel council charged with leading the rebellion against the Terran Empire, which included Sarek and Voq. Listening to the mirror Voq speak was enough to trigger dormant memories in Tyler's mind, which awakened the truth in him that he was, in fact, Voq himself. Having reasserted his Klingon personality, he physically attacked his counterpart, but was defeated in combat. Since Sarek confirmed, through the use of a mind meld, that Burnham's motives were altruistic, she and Tyler were allowed to return to the Shenzhou, where Burnham confronted him in her quarters and learned the truth of his Klingon origins. "Tyler" attacked Burnham and planned to kill her but was thwarted by the intervention of Saru. Facing execution for apparently attempting to kill the ship's captain, per standard Empire orders, "Tyler" was sentenced to be beamed into space. Burnham, however, operated the transporter herself to ensure that "Tyler" could be quickly beamed aboard the Discovery. She had hidden stolen files about the USS Defiant on his person, which the Discovery crew planned to use to return to their universe. Once aboard, "Tyler" claimed that Burnham should have left him to die, but Saru quickly reminded him that was not in the tradition of Starfleet. (DIS: "The Wolf Inside")
"Tyler" was confined to the medical bay where Dr. Pollard and her medical team examined him; their scans confirmed that he was Human, even with "Tyler" displaying enhanced strength. "Tyler" then asked Saru for help, for which Saru turned to L'Rell, who revealed that Voq was given Tyler's personalities and memories in order to infiltrate Starfleet as a spy. Upon seeing "Tyler" in pain, L'Rell agreed to help him, performing a procedure which removed Voq's personality and left Tyler's personality in control – L'Rell marking Voq's passing with the traditional Klingon ritual. (DIS: "Vaulting Ambition")
Upon his recovery and Discovery's return to their universe, Tyler, with Voq's memories, told Saru how Voq agreed to undergo the choH'a'. Despite this revelation, Tyler was accepted by the his fellow crewmates such as Cadet Tilly and Lt. Detmer. (DIS: "The War Without, The War Within")
Background information Edit
"Ash Tyler" was portrayed by actor Shazad Latif.
Originally, CBS announced in December 2016 that T'Kuvma's protégé, then named "Kol", was to be played by Shazad Latif. In the final version of the series, Latif went on to portray "Ash Tyler" and T'Kuvma's protégé was renamed "Voq", while the name Kol was given to a leader of a rival Great House.  Ultimately, however, it turned out that the character of Ash Tyler was actually a construct for Voq to infiltrate Human society.