Lucy Stillman (born in 1988) was hired by Abstergo Industries for her thesis project which related to the work that Abstergo wished to conduct, the Animus was mostly her brainchild. She was confirmed to be a spy for the Assassins working inside the Templar faction for information. She makes at least one attempt to get help for both herself and Desmond during Assassin's Creed.
Lucy, working for Abstergo Industries, helped answer questions for Desmond in between sessions in the Animus, though only to a certain extent as there were certain things she couldn't reveal. As they spoke with each other, Lucy grew less formal and told Desmond stories such as how her death was ordered by Abstergo in the past. Later on, Lucy leaves behind codes in Desmond's wardrobe, allowing him to leave his room for the first time. Eventually Lucy reveals to Desmond that she is also an assassin and that she was spying on Abstergo to learn their plans. When Desmond's death was ordered by Abstergo after finishing inside the Animus, Lucy intervened and persuaded Abstergo to keep him for more testing, but in return she had to leave him behind in the lab.
Assassin's Creed 2
A few hours after the events of the first game, Lucy returned to the lab with blood on her shirt. Lucy told Desmond to quickly use the Animus to gather the memory core data, and the two then rushed to leave the Abstergo facility. Leaving the facility by car with Desmond in the trunk, they arrived at the new hideout. Lucy tells Desmond that she wants to train him as an assassin by using the Animus and the bleeding effect, and Shaun and Rebecca, her fellow assassins, quickly introduced themselves to Desmond. The next day after using the Animus, Desmond told Lucy that he will protect her. Eventually, Abstergo invaded the hideout and the alarm went off. Lucy gave Desmond a Hidden Blade like Ezio, which he used to fight the Abstergo guards. She warned Desmond about Warren Vidic, since he saved her when she was ordered for death. They soon encounter Vidic, who yells at Lucy for betraying him. After Desmond disposes of the guards, Lucy stops Desmond from going after him, saying that he will be brought to justice eventually. Since their hideout was discovered, the assassins left for a new one so that they could continue with Desmond's training inside the Animus.
Assassin's Creed: Brotherhood
They arrive at Monteriggioni and decide to set their hideout underneath the Villa Auditore to avoid detection from Abstergo. She and Desmond travel through a hidden entrance, and after much friendly banter, they reach the Sanctuary underneath the Villa and open the doors for the other two. After bringing power back to the Sanctuary, Desmond uses the Animus once again. Between sessions, Desmond would sometimes take a break and speak with Lucy and the other assassins. They eventually discover that the Apple of Eden was hidden under the Roman Colosseum. After they arrive, Desmond opens up a way for the rest of them to follow, and they reach the Santa Maria Aracoeli. Desmond activates an elevator, and the assassins use it to reach the Colosseum Vault. When Desmond tries to grab the Apple of Eden, Juno, a member of the First Civilization, takes control of him and freezes Lucy and the other assassins in place. She speaks to Desmond and forces him to stab Lucy in the abdomen, and they both collapse. The game ends with both character's fates unknown.
Assassin's Creed: Revelations
It is revealed that Lucy has died after being stabbed in the previous game.
In "The Lost Archive" DLC, it is revealed that Lucy was with the Templars all along. She was ordered by Vidic to plan their escape from Abstergo in order to get close to the Assassins. Once the Apple of Eden was discovered, she was to bring it to him. This revelation is the reason why Juno controlled Desmond and stabbed Lucy, explaining that "the cross darkens the horizon".