Lucy Stillman is an assistant researcher at Abstergo Industries and one of the two characters in the "present" that you interact with playing as Desmond Miles in the game. Unlike Dr. Vidic, she is friendly and approachable, and she is known to care for Desmond Miles, pulling him out of the Animus when she feels he in it too long. Her e-mail can be hacked to reveal more background story surrounding Abstergo and their ties to the events surrounding the present timeline of the game. Through conversations and a few events, she is confirmed to be an assassin. Lucy's appearance is modeled after actress Kristen Bell, who also the voice actress.
Throughout the the game, Desmond has several opportunities to converse with Lucy in order to gain more information about his situation as well as to get to know her better. These short conversations occur at the end of each memory segment and after Dr. Vidic has left the area. At some point, Lucy begins to tell Desmond little bits of her background. She reveals that she was ostracized for her scientific beliefs and theories in her college career. After her graduation, she couldn't find any good jobs at any scientific organizations. Reaching her wits end, she received a call from Dr. Vidic who claimed he agreed with her scientific theories and invited her to work for Abstergo in order to prove them true. As to what those theories are has not been revealed, and will most likely be answered in a sequel.
Lucy hints on several occasions that she too is being held captive by Abstergo. From reading her emails, the veracity of her claims appear to be legitimate. Furthermore, in her emails Desmond reads a few messages about Lucy's fellow employee and friend Leila Marino, who allegedly committed suicide. However, the information concerning Leila's death was considered classified and therefore not released to Lucy, so the details are still unknown. It also becomes clear that Lucy's reason for refusing to discuss the details of Abstergo's goals is for her own safety. Lucy left the codes to access the door leading out of Desmond's room in his closet, as well as her access pen on the Animus so that he could access her email. By doing so, she allowed Desmond to acquire a better understanding of Abstergo's background information. At the end of one conversation, Lucy symbolically pulls back her left ring finger to make it appear like Altaïr's severed finger, telling a dumbfounded Desmond to "have faith."
Towards the end of the game, Desmond comes across a set deleted emails on Lucy's computer. In these emails, he reads in conversation in which Lucy said she is stalling Dr. Vidic from finding the artifacts and requesting a rescue. A bizarre reply with arbitrarily capitalized letters decode a secret message that says "WE WILL BE THERE SOON." Using a newly acquired Eagle's Vision, Desmond sees Lucy with a blue aura, thus confirming she's an ally and an assassin.