Carly Spencer, whose full name is Carly Brooks-Spencer, is a human character introduced in the Transformers: Dark of the Moon film. She is a young, accomplished British woman who first met protagonist Sam Witwicky at the White House on the day he received a medal from the President for his prior heroics. At the time of their meeting, she was working at the British embassy. The two were immediately attracted to each other despite an awkward start and soon began a romantic relationship, with Carly moving into an apartment with Sam.

Carly is characterized as smart, confident, and savvy, possessing an adept calm that allows her to handle even life-threatening situations with composure. She is joyful and hard-working but can also be stubborn and prone to losing her temper when things do not go her way or when she is personally upset. Her core motivation is her relationship with Sam, whom she encourages and appreciates for who he is. She provides him with emotional support, exemplified by her gift of a "lucky bunny" to serve as a positive motivator for his job interviews. While she believes in his heroic past, she also wishes for a stable and safe life with him, expressing frustration when his involvement with the Autobots takes precedence over their relationship.

As the story progresses, Carly secures a job as an assistant curator or secretary for Dylan Gould, a wealthy CEO and car enthusiast. Her role in the narrative deepens when Gould is revealed to be a Decepticon collaborator. After Sentinel Prime betrays the Autobots, Carly is taken hostage by Gould. The expensive Mercedes-Benz SLS AMG given to her by her employer is revealed to be the Decepticon Soundwave, who traps her inside to force Sam's cooperation. She is ultimately taken to Chicago, where the Decepticons plan to transport their home planet of Cybertron to Earth.

Despite being a hostage, Carly demonstrates significant bravery and resourcefulness. After being rescued by Sam, she actively participates in the battle for Chicago, enduring the destruction and chaos of the warzone. She proves herself a capable trickster by using psychological manipulation against Megatron. Observing that Megatron is being treated as subordinate by Sentinel Prime, Carly confronts the Decepticon leader, taunting him that he will become "Sentinel's bitch." This calculated manipulation enrages Megatron but also leads him to realize the truth of her words, prompting him to turn against Sentinel and allowing Optimus Prime to defeat both Decepticon leaders, effectively ending the battle. Following the conflict, she and Sam finally confess their love for each other.

Carly's key relationships are central to her character. Her bond with Sam Witwicky is the driving force of her personal journey, moving from a supportive girlfriend to a partner who fights alongside him. She has a tense but non-adversarial relationship with her boss, Dylan Gould, whose true villainous nature transforms him into her captor. Among the Transformers, she has a strained relationship with the small, mischievous robots Wheelie and Brains, who invade her privacy. In contrast, she develops a deep affection for Sam's Autobot guardian, Bumblebee, even stating that she loves the car. A notable ability displayed by Carly is her psychological acuity and skill in manipulation, which she uses to exploit Megatron's pride and turn the tide of the final battle. Additionally, she exhibits remarkable endurance and bravery, which one source attributes to her having been raised in a military family.