This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
Vice Admiral Katrina "Kat" Cornwell was a Human female Starfleet flag officer who lived during the mid-23rd century. She was a psychiatrist by training, and had experience treating PTSD. (DIS: "Choose Your Pain", "Lethe", "Into the Forest I Go")
In November of 2256, she informed Lorca, captain of the USS Discovery, about a concurrent Klingon attack on Corvan II and ordered him to protect that planetoid. (DIS: "The Butcher's Knife Cares Not for the Lamb's Cry")
In December of 2256, she attended a strategy briefing with Lorca, Terral, and three other admirals. She ordered Lorca to restrict use of the spore drive, since the Klingons might have learned of it and might target the Discovery. (DIS: "Choose Your Pain")
A week later, she visited the Discovery after Lorca launched an unauthorized rescue mission for Ambassador Sarek, whose ship had been disabled on the way to a diplomatic mission by an explosion that had deliberately been caused by an aide who had turned out to be a logic extremist. Cornwell and Lorca attempted to rekindle their romantic relationship. However, Cornwell made the decision to declare Lorca unfit for command after she tenderly awoke him the morning after, and he responded with a phaser in one hand and his other at her throat (due to lingering trauma relating to Lorca having recently served time in a Klingon prison; she was quite upset he kept a phaser under his pillow).
However, Sarek was too injured to attend his diplomatic mission, a meeting with a supposed dissenting Klingon faction which could end the war, so Cornwell took Sarek's place, planning to recommend Lorca be stripped of command afterwards. The mission turned out to be a trap orchestrated by Klingon General Kol, and Cornwell was captured by Dennas and Ujilli. (DIS: "Lethe")
Sometime later, Cornwell was kept prisoner by Kol aboard the Sarcophagus. Despite weeks of torture, she refused to give up any information. L'Rell offered her services to Kol as an interrogator in exchange for a cloaking device, but this was a ruse in order to get close to Cornwell. The imprisoned admiral met L'Rell when the latter arrived in her cell. At L'Rell's insistence, Cornwell screamed, which L'Rell used to distract a guard away from the area. The admiral also learned that L'Rell wanted to defect to the Federation in exchange for helping her escape, the Klingon having grown disillusioned with how easy her people fell in line with Kol.
However, during the escape, L'Rell and Cornwell were confronted by Kol. Realizing they were unlikely to escape, Cornwell grabbed L'Rell's blade and attacked her, in order to maintain the illusion in front of Kol's men. Afterward, L'Rell hid the unconscious Cornwell in a side room – a room which, to L'Rell's horror, was filled with corpses. (DIS: "Si Vis Pacem, Para Bellum")
When Ash Tyler and Michael Burnham infiltrated the Sarcophagus to place sensors on the ship to allow Discovery to detect the Klingon ship while it was cloaked, the pair picked up Cornwell's life signs and found her in the side room where she had been placed earlier. Discovering L'Rell present in the same room, Tyler began experiencing intense flashbacks, and was unable to continue the mission. Burnham went on to complete the placement of the sensor on the bridge, while Cornwell tried to bring Tyler back to reality.
After Burnham successfully completed the placement of the sensors, Cornwell was beamed back to the Discovery along with Tyler, Burnham, and L'Rell. Cornwell was sent by emergency medical shuttle to Starbase 46 where she underwent surgery. She was expected to make a total recovery. (DIS: "Into the Forest I Go")
Background information Edit
Cornwell was portrayed by actress Jayne Brook.
The first name "Katrina" was initially only mentioned in the Netflix German voice dubbing list for "The Butcher's Knife Cares Not for the Lamb's Cry", but was first spoken in the following episode, "Choose Your Pain".
In Episode 4 of After Trek, shown after "Choose Your Pain", showrunners Gretchen Berg and Aaron Harberts mentioned that Cornwell is a psychiatrist by training, who is not only old friends with Lorca, but is assessing him as she supervises him, and is worried about him.
When asked how Cornwell would handle Saru's "break from reality" in "Si Vis Pacem, Para Bellum", Jayne Brook commented that the admiral would try to talk Saru down slowly rather than threaten to remove him from serving on his ship. Brook went on to say, "I think that she would give him time. I think she's a measured person who gives people the space, or wants to give them the space, to heal, and to observe herself. I think she would observe him and give him time." (After Trek: "Episode 7")