Lieutenant Sharon "Boomer" Valerii is a Cylon sleeper agent programmed to believe that she is human. 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 that killed all the members on the mining colony. In the time before the mini-series, Boomer and Chief Galen Tyrol are romantically involved, despite their relationship being against military protocol (she is an officer, he is enlisted). Due to her programming, she performs a number of sabotage actions (including bombing the Galactica's water tanks), which she has no conscious memory of, although in time she begins to suspect something may be wrong with her. She asks Gaius Baltar to administer a blood test to find out if she is a Cylon. Although his test determines that she is in fact a Cylon, for his own reasons he tells her she is not. She is greatly relieved for a while, but over time becomes increasingly aware that something is wrong with her, and even attempts suicide. Tyrol hides evidence that would implicate her in the water bombing, but later, suspicious events cause him to break off his relationship with her.

Sharon's programming ultimately takes over and she shoots Commander William Adama, putting him in a coma. She and Tyrol (suspected of being a Cylon because of his relationship with her) are put in the brig, interrogated and brutalized by Colonel Saul Tigh, who has taken command of Galactica in Adama's incapacity. Gaius Baltar ultimately forces her to tell him how many other Cylons were onboard by threatening Tyrol's life. Sharon informs Gaius that there are 8 Cylons in the fleet. This information has not been verified and could be unreliable (Sharon could've picked any number to spare the Chief's life- interestingly, 8 is her Cylon designation). She is shot and killed by Specialist Cally, who blames her for Tyrol's imprisonment; Cally herself is in love with Tyrol as is made clear in later episodes. Sharon dies in Tyrol's arms. Her last words are, "I love you, Chief."

In the episode "Downloaded", it is revealed that Boomer's consciousness has been downloaded into a new body, yet she resists being identified with the Cylon cause. She is viewed as a hero among the Cylons, but in this episode she and fellow Cylon Caprica-Six, transformed by their love for humans in previous incarnations, decide to try to persuade the Cylons that some of their actions against mankind are wrong. Because of her and Six's actions, she is, for a time, threatened with "being boxed," that is, not allowed to downloaded into a new body when the current one is killed/dies. She is among the Cylons that find New Caprica and tries to show that humans and Cylons can co-exist, and functions as a member of the Cylon governing council that controls the puppet human government under President Gaius Baltar. She seems to have accepted herself as a Cylon and is fully integrated back into their society.

On a Cylon Base Ship, Boomer has been seen attempting to care for Athena's hybrid child Hera, who is mysteriously ill. She is frustrated by the child's deteriorating health and their inability to bond even though Boomer has "tried everything". When Athena arrives on the Base Ship and determines that Hera would best be treated by a human doctor, Boomer accuses her of having planned to take Hera back to Galactica from the start. Boomer says that the failure of New Caprica has taught her that humans and Cylons can't co-exist and that they should go their separate ways, and thinks that Brother Cavil may be right that the Cylons aren't supposed to have children. She threatens to snap Hera's neck, but her own neck is snapped by Caprica-Six for endangering the future of the Cylon race.

When we next see Boomer, she has done the unthinkable and voted against her own model line. The Cylons have become divided among model lines over various issue -the boxing of the Threes, learning the identities of the Final Five Cylons, and lobotomizing the raiders- and Boomer sided with the Ones, Fours and Fives (1,4,5), while the rest of the Eights sided with the Sixes and Twos. This unprecedented move shocked her fellow Cylons and left the other Eights feeling betrayed. Boomer has been involved in a romantic relationship with the One known as Cavil. It's unknown if she was among those massacred by Natalie and her Centurions, only that she was with the rest at the resurrection tanks after. (Boomer, at this point, seems fractured beyond repair over the events in her life. Eights are viewed as weak by all other Cylons, and each decision she makes is considered a sign of that- no matter which side the Eights take, they're sneered at. It seems this has begun to take a toll on all Eights, especially Boomer and Athena.) When the 1,4,5's retaliate against Natalie and her rebel Cylons by attacking them outside the range of a resurrection ship, Boomer expresses regret, doubt, and pain at the death of her " own sisters." 

Boomer and Cavil travel to the Resurrection Hub to unbox D'Anna. After D'Anna kills Cavil, Boomer leaves the Resurrection Ship before it is destroyed. Boomer becomes a sort of pupil to Cavil, and is the only Cylon other than Cavil who seems to be aware of Ellen's existence and true nature. Boomer seems to show interest in Ellen's view of Cylons and humans, and eventually helps Ellen escape from the basestar. Boomer takes them to Galactica, where she is promptly arrested and taken to the brig.

The Cylon rebels demand that Boomer be put on trial for treason. With resurrection no longer a possibility, death would be permanent. While in the brig, Boomer is visited by Tyrol and she confesses that she still loves him. She shares with him projections of the home she had always wanted to have with him, complete with a daughter (Dionne) and wedding photos. When it becomes apparent that the Colonials and Cylons have no intention of giving Boomer a fair trial and will simply execute her, Tyrol bludgeons another Eight and switches her with Boomer. Boomer attacks Athena in the head, binds and gags her, then locks her in a stall. Boomer puts on Athena's clothes and is preparing to leave the head when Helo finds her. Believing her to be his wife, he tries to seduce her before a six-day mission. Initially, Boomer resists but gives in when it seems Helo is beginning to sense something is wrong.

Boomer makes her way to daycare and she takes Hera. Hera is drugged and placed in a provisions supply case, and Tyrol (unaware of what's happened) helps Boomer load the case onto a raptor. Boomer asks Tyrol to come with her but he declines, and she tells him that she loves him. Boomer is nearly prevented from leaving Galactica, but she warns Adama that Hera is with her and is able to escape Galactica. When she jumps away, she is too close to the battlestar and the spatial disruption causes severe damage to the ship.

The others aboard Galactica realize Boomer was deceiving them the whole time and only ever intended to kidnap Hera. While jumping to The Colony, Boomer becomes increasingly aggravated by Hera's cries. In a fit of rage, Boomer tries to administer an adult dose of a sedative to Hera, but instead tells Hera about how her projections. While projecting the house again, Hera surprises Boomer by revealing that she can see Boomer's projection. Boomer takes her to her (imaginary) daughter's room. By the time Boomer and Hera arrive at The Colony, they have bonded. Boomer is reluctant to hand Hera over to Cavil and Hera resumes crying. When it is decided that Hera will be experimented on, Boomer voices concern for Hera and flat out opposes the experiments. But the 145s decide to continue.

When The Colony is attacked by the Colonials and Rebels, Boomer kills the Simon examining Hera, grabs her and runs. Boomer takes Hera to the rescue party - consisting of Athena, Helo, Starbuck, and some marines - and gives Hera back to Athena. She admits that it doesn't make up for what she's done, that she made her choice, and that it was her last. Boomer tells Athena to tell the Admiral that she owed him one and then nods to Athena. Athena opens fire and Boomer is permanently killed.