- One of Caulfield's instructors at the Ghost Academy(src)
Caulfield entered the Ghost Program in 2492, and remained in the program when it came under the auspecies of the Terran Dominion. Over her entire time with the program, she was never given a single assignment. During the Second Great War, she was pulled out of the Ghost Academy and sent to a remote location. She remained in hiding during the End War. She never understood why she'd never been deployed, but after the war's end, she reckoned that when the next war came, she'd be part of it. Only that war had never come, leaving her with a mix of feelings. On one hand, she felt like she'd been wasted. On the other, many ghosts had been killed during the wars of the Koprulu sector, and she had some relief that she'd been spared the carnage. This gave her some survivor's guilt however, as she pondered how many ghosts had been deployed in her place. Caulfield felt alone in the world, and immensely disliked her fellow ghosts, but quickly befriended Ulavu, an exiled protoss researcher aiding the Ghost Program.
Six years after the End War, Caulfield was in Augustgrad in the quarters reserved for ghosts, partnered with Ulavu. As the sixth anniversary of the war's end loomed, she received her requested discharge from the Ghost Program. Ulavu sensed her thoughts via telepathy, but she assured him that nothing was wrong, and reflected on everything she had (and hadn't) done as a ghost up to this point. Her thoughts were interrupted via her telepathic connection with Ulavu—he'd left the establishment without a military escort, and found himself in Dante's Circle, confronted by a group of Chau Sarans who weren't happy to see a protoss after the glassing of their world over a decade ago. She found her way to the bar, but saw that one of the patrons was drunkenly harassing Ulavu. Caulfield attempted to diffuse the situation, but was unable. Seeing no alternative, she threw a glass of liquid at the patron, and used her pyrokinesis to boil the liquid just enough to stun him without causing permanent damage. The two then returned to the Ghost Academy.
Later, Ulavu was tasked by Emperor Valerian Mengsk to accompany him to Gystt, where Overqueen Zagara requested the aid of the Dominion against a Daelaam fleet commanded by Hierarch Artanis. She followed her friend to the field, and in spite of her resignation Ulavu requested to Colonel Abram Cruikshank that Caulfield be assigned to the survey team Ulavu was put on. Cruikshank reluctantly accepted, and Caulfield and Ulavu were partnered with Foster "Whist" Cray, a marine sergeant, Dennis "Dizz" Halkman, a reaper, and Doctor Erin Wyland, a xenobiologist. Their goal was to see if Gystt was hiding any secrets, and if Zagara's intentions of peace were genuine.
Caulfield and her allies landed on the surface and began scouting the alien landscape. However, soon they were ambushed by a small group of zerg. When they attempted to rectify this with Zagara, she told Emperor Valerian that they had crossed a point of control between two broodmothers, and if they moved on they would go unimpeded. However, soon they arrived at a strange structure made of the local trees, and were once again attacked by local zerg. Caulfield and her squad were forced into the structure, and fought their way in.
Once inside, Caulfield felt a strange psionic buzzing from within the structure, and something that felt almost mechanical. However, once further in they discovered chitha, strange, hydralisk-like zerg that emitted strong psionic energy. These zerg focused their psionic attacks on Caulfield, immediately knocking her unconscious.
Caulfield awoke later, finding her squad had defeated the chitha and began to move into the central chamber. There they found strange creatures in sacs of liquid, which looked similar to the chitha. After the group was assaulted again, Tanya used her pyrokinesis to destroy the remaining chitha in the room. Coming up with a new plan, Dizz set up grenades and flammable zerg corpses through the room and used Caulfield's pyrokinesis to ignite them, killing the sac creatures. When setting up the explosives however, Caulfield noticed that the zerg from earlier were killed by precise cuts, similar to a psionic blade. She then concluded that it was Ulavu, and he had lied to her about being a simple researcher. This caused her to feel deeply betrayed
This however caused a situation, when it was revealed the creatures, known as the adostra, were a hybrid of xel'naga and zerg essence, and were peaceful. When Valerian told them of this and that Zagara had no knowledge of the chitha, he send a force to reinforce the protoss at the second chitha point, and ordered Caulfield's group to move to the third nest. While they were regrouping, Caulfield was confronted by Ulavu, who noticed she was becoming distant. She told him about what she discovered, and he revealed he was a Nerazim sent to study the terrans in order to ascertain their motivations and discover a telekinetic psychic to aid in research on his warp disk, but befriended her. While it did not heal the feeling of betrayal, Caulfield at the very least decided that it was a start to helping their friendship.
The group boarded a dropship for the third nest, but soon the ship was attacked and incapacitated by mutalisks, who were being drawn toward a psi emitter dropped by the Dominion forces in orbit. Caulfield and her group defeated the mutalisks, freeing their dropship before the psi emitter site was destroyed by a nuclear missile. The dropship landed, too damaged to go further, and the group made their way to the third nest on foot.
At the third nest, Dizz pointed out a hole in the ceiling, hinting that she may want to escape to avoid capture by the zerg. Caulfield considered this, but encouraging words from Ulvau kept her on her path, and the group found more adostra and a strange communicator, a mix of terran communications and zerg biomechanics. They pieced together that Abathur, the zerg evolution master, was using this to control the chitha, and that he desired the terran and protoss forces to destroy them. The group was attacked, but thanks to the quick thinking of Erin Wyland the zerg were incinerated by her flamethrower. Caulfield and Ulavu pulled her from the wreckage and were able to give her basic treatment before her burns became too severe.
Caulfield and her group were then rescued by a protoss force and Overqueen Zagara, who was pleased to see them alive and her adostra in good shape. Thinking through where Abathur may be, Valerian requested that Zagara meet aboard the Hyperion to discuss the situation further, and Caulfield and her team go with Zagara aboard her leviathan. However, Valerian, knowing that Abathur would use the chitha to jam outbound communications, created a communication's device in the shape of a C-10 rifle, and gave it to Caulfield. The group then entered the leviathan, and found Abathur inside.
Abathur revealed he was behind the chitha and the attempts to kill the adostra, and that he wanted war in order to strengthen the swarm. Zagara attacked him, and Abathur's allied queen Mukav attacked Caulfield and her party. She was quickly killed by Whist and Ulav's assault. However, they were quickly overrun by chithas. In a last ditch attack, Caulfield overloaded her neural inhibitor, and unleashed a pyrokinetic firestorm that slew all chitha in the room. She then immediately passed out.
Caulfield awoke in the medical bay of the Hyperion, with Ulavu by her side. He revealed that they had defeated the chitha, and that Abathur was still incapacitated by Zagara, but still alive. He also stated that he was returning to the Daelaam, as if his true motivations were revealed it may cause feelings of betrayal in the Dominion. Caulfield said his secret was safe, and Cruikshank confronted them, stating that he held Ulavu responsible for Wyland's injuries. Caulfield was curious as to why he did not tell him that he was responsible for Wyland's injuries, rather that he saved her, and he stated that Cruikshank was technically correct, that he did cause her first degree burns with his warp blade. Caulfield for her part revealed she would not resign from the Ghost Program, and was going back into duty. The two agreed that they would maintain their friendship.
Tanya Caulfield is a strong psychic, but prideful and quick to anger. Usual ghost methods of having a word to calm them do not work with her, and when she uses her pyrokinetic abilities she flies into a fit of rage. Caulfield is antisocial, disliking many of the terrans she worked with, but growing a strong attachment to Ulavu, considering him a close friend she trusted. While prideful, was not about mending fences after she and he fell out. Caulfield has a strong need to feel useful, a need that caused her to tender her resignation from the ghost program after she was deemed too important to be put on the frontlines.