A member of the Special Tactics And Rescue Service/Squad (S.T.A.R.S.) Alpha team in Raccoon City, Jill was a former member of Delta Force. In Resident Evil 5 Jill is part of the Bioterrorism Security Assessment Alliance.
On July 23 1998, the S.T.A.R.S. Bravo team was deployed to the outskirts of Raccoon City located in the Arklay Mountains after evidence of grisly, cannibalistic murders. A day later, after contact with Bravo team was lost, the Alpha team was sent to find out what happened to their comrades. Soon after their arrival, the team found the destroyed remains of Bravo team's helicopter and were quickly attacked by vicious Cerberi; canines infected with the t-virus. After witnessing the death of fellow Alpha team member Joseph Frost, the team was abandoned by their helicopter pilot, Brad Vickers and the remaining members fled to a nearby mansion, the site of the Arklay Research Facility.
Jill, along with fellow survivors Chris Redfield, Barry Burton, and Albert Wesker, assumed the mansion to be deserted. They would quickly discover the various beasts and monsters inhabiting the mansion, as well as the remains of their fallen Bravo Team compatriots. Through her cooperation with Barry, Jill was able to discover that the mansion was actually a front for a top-secret research facility owned by the Umbrella Corporation. They also came to find that Wesker was a double agent working for Umbrella and had planned the whole operation as an experiment to test out Umbrella's t-virus B.O.W.s and to destroy the S.T.A.R.S. team. However, after Wesker was seemingly killed by one of his own creations, Jill and Barry escaped, along with Chris Redfield and surviving Bravo team member Rebecca Chambers, leaving behind the ruins of the Estate.
After their return to Raccoon City, S.T.A.R.S. request for a full-scale investigation of Umbrella's activities was denied by their superior, Chief Brian Irons of the Raccoon Police Department (RPD). With no support from the American government, Chris, Barry, Jill, and Rebecca decided to travel to Europe and investigate Umbrella's main headquarters themselves. Chris went ahead and travelled to Europe by himself by the end of August, while Barry went to Canada to relocate his family first. Jill decided to stay in Raccoon in order to investigate the whereabouts of Umbrella's Underground Facility before joining her comrades.
However, two months after the mansion incident, her investigation was ruined when Raccoon City was infected with the t-Virus. During her escape, she witnessed the death of Alpha team's pilot, Brad Vickers, at the hands of "the Nemesis", a Tyrant sent by Umbrella to assassinate the remaining S.T.A.R.S. members. Jill managed to thwart Nemesis temporarily and met with Carlos Oliveira, a surviving member of the Umbrella Biohazard Countermeasure Service (U.B.C.S.) along with Nicholai Ginovaef and Mikhail Victor. However, Mikhail gave his life protecting Jill and Carlos from Nemesis while Nicholai was presumed dead after an accident. Jill and Carlos ended up in a nearby clock tower after their cable car collided off-course. During the following battle with Nemesis, Jill was infected with the t-Virus and was taken to the clock tower's church by Carlos, where she lay there in a semi-coma.
Two days later (on the night of October 1st), Jill recovered after being administered with a serum by Carlos, and continued her search for escape. Soon afterwards, she discovered another one of Umbrella's secret research facilities hidden behind a park. In this lab, she confronted Nicholai for the last time (it seemed that she had a bounty on her head that he wished to collect), and fought with Nemesis once again. Jill successfully destroyed Nemesis with a top-secret weapon that was transported to the facility a few days earlier and managed to escape from the city in a helicopter with Carlos. As they flew away, Jill witnessed the annihilation of Raccoon City on the order of the U.S. government.
Jill would go on to become involved in various anti-Umbrella groups and activities until Umbrella's closure.
In 2003, Jill and Chris went with an anti-B.O.W. group into Russia in search of an Umbrella base. When they arrived, they found that the bio-weapons in the facility had been released.
After a battle to claim the surface, the two went down on a large cargo lift to the Umbrella base where they fought against the escaped bio-weapons.
Eventually, they made it to a laboratory containing the T-A.L.O.S.-project, their mission objective being to destroy it.
After a chaotic battle, they managed to defeat it and and the team re-assembled and guarded the base in waiting for the Russian government arrival.
*Note: IF YOU HAVE NOT BEAT RESIDENT EVIL 5, DON'T CONTINUE!!!!!!*
After the collapse of Umbrella, rivals and terrorist groups managed to get their hands on Umbrella's projects. With the growing fear of a bio-terror holocaust in Africa, Chris and Jill joined a Bioterror Assessment group partly to locate Wesker who was revealed to be alive when Chris fought him while searching for his sister in Resident Evil: Code Veronica. In 2006, a tip led Chris and Jill to arrest Ozwell E. Spencer, one of the key founders of Umbrella, but when they located him he was already killed by Wesker. Chris and Jill opened fire but were easily taken down by Wesker's superhuman strength. With Wesker ready to deliver a fatal blow to Chris, Jill selflessly tackles Wesker (releasing his grasp on Chris) out the window and descended down the cliffside with Wesker firmly in her grasp.
Her body was never found and she was presumed dead. In truth, however she had survived, but was badly injured and unconscious. Wesker then took Jill, and intended to use her as one of the first test subjects in his Uroboros Project. Placing her in cryogenic sleep, Wesker discovered an abnormality within Jill's body: a remnant of the t-virus infection from Raccoon City, which had been reactivated by the cryogenic stasis. As the virus disappeared, it left in its place powerful antibodies, which Wesker then used in order to help make the Uroboros less poisonous, thus giving human subjects a better chance of surviving infection. As a result of the antibody extraction, Jill's hair became a light blond color, and her skin became paler due to etoiliation. After using her to further his virus-development, Wesker then decided to use her as a potential super soldier. His method was to use "P30", a Progenitor-virus based chemical, which granted the user immense strength and speed, but also made them susceptible to control. While P30 had potential, it also lasted for only a short time, due to the body metabolizing it quickly. To remedy this Wesker placed a mechanism on Jill's chest to release a steady stream of P30 into her, in order to keep her under his control, while she hid behind a cloak and bird shaped mask.
When Chris Redfield and Sheva Alomar found Ricardo Irving, a weapons dealer they had been tracking down, a cloaked masked figure intervened and rescued him. Previously, Chris found files which included pictures of a blonde woman in a pod who looked like Jill Valentine, giving him hope for her survival. Chris and Sheva searched for Jill, and eventually came across the pod they had seen within the file, though it was now empty. They were contacted by Wesker's ally Excella Gionne who claimed that she didn't know where Jill was. Not believing her, they decide to track her down. When they encounter Gionne in the flesh, the Masked Lady interrupts and nearly overpowers the two agents, despite being outnumbered.
A stray shot knocks away her mask, and Wesker pulls back her hood, revealing her to be Jill Valentine herself. Chris tries to talk to Jill but, under Wesker's control, she attacks him and Sheva with a flurry of hand to hand moves. This starts a battle, where the player has to avoid Wesker for 7 minutes or severely damage him enough to make him flee. During this time, Wesker can avoid virtually any attack thrown at him unless Chris is hiding at the time, while Jill can still be damaged and knocked down. If the player kills Jill, they get a game over screen reading "Your Partner Died", the same as if their AI partner was killed. Wesker gets ready to leave, but upon hearing Chris call Jill's name, Jill resists Wesker’s control. He notes down the resistance, calling it fascinating to be happening at such a late stage and leaves her in great pain. A mechanism on her chest, responsible for injecting her with the P30 serum, is exposed at this point and has to be removed. Chris can try to pull off the device, or it can be shot off. (at a risk though, as Jill dies easily) Released from Wesker’s control but severely weakened, Jill apologizes for her actions, stating that she had no control but was fully aware of everything she did. She tells the pair that they have to leave her behind and stop Wesker. Chris objects, but grudgingly agrees when Jill says “Don’t you trust your partner?” After the pair leaves through an elevator, Jill finally says. “You’re our only hope of surviving this.”
Later, after Sheva and Chris defeat Uroboros Aheri, Jill calls Chris on his video phone and passes along some vital information: Wesker's superhuman speed and strength come from the virus within him, a virus which is kept in balance by a serum he must be injected with. If he can be injected with an overdose of the serum, it should severely damage his body and weaken his abilities. As it happens, the two of them had just happened to come into possession of a vial of the serum earlier in the day, and it is only through application of this serum that Wesker is forced to retreat.
Jill does not appear again until the very end of the game when Wesker falls into the molten lava. Jill, riding in a BSAA helicopter piloted by Josh Stone, throws a ladder out to Chris and Sheva. They climb into the chopper, Chris barely grabbing on before the ground melts into magma. Just as they climb inside the cabin, Wesker desperately reaches out with his mutated arm and immobilizes the helicopter, attempting to bring his most hated enemies with him as he dies. Jill tells Chris and Sheva to use the weapons stored in the the cabin, a pair of RPG-7s. They destroy Wesker and release his grip on the helicopter.
In the final cutscene in the chopper, Jill simply says "Finally...." in response to Chris saying "It's over...".