Number Eight is a female humanoid Cylon model on the television series Battlestar Galactica, a reimagining of the classic show of the same name. She is portrayed by Grace Park. Two prominent Number Eight copies serving as Galactica pilots are Sharon Valerii and Sharon Agathon, using the call signs "Boomer" and "Athena", respectively.

Many Number Eight copies have been seen in various roles, including leadership positions; two are featured as central characters — Boomer and Athena. Lieutenant Junior Grade Sharon "Boomer" Valerii is a Cylon sleeper agent programmed to believe she is human. For example, she has been implanted with false memories of being raised on Troy by her parents Katherine and Abraham Valerii, who supposedly died in an accident killing all the members on the mining colony.

Whether or not Katherine and Abraham actually existed is unknown. In the time before the mini-series, Boomer and Galen Tyrol are romantically involved, despite the fact their relationship is against military protocol (she is an officer, he's enlisted). Due to her programming, she performs a number of sabotage actions (including bombing the Galactica's water tanks) but has no conscious memory of these actions.

In time she begins to suspect something may be wrong with her. She even asks Gaius Baltar to administer a blood test to find out if she is a Cylon. Although his test demonstrates she is in fact a Cylon, he decides to not inform anyone out of fear she will kill him.

She is greatly relieved for a while, but over time becomes increasingly aware something is wrong with her, and even attempts suicide. However, her programming does not allow her to kill herself. Tyrol hides evidence implicating her in the water bombing, but later suspicious events cause him to break off his relationship with her.

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