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Jemmy O'Neill is the daughter of Giana O'Neill and her first husband. Jemmy's stepfather Simon O'Neill later raises her as his daughter together with Giana.

During the attack on the Colonies, she was with her stepfather on Gemenon and would escape aboard the container ship Cybele. Several days later, she was reunited with her mother, now a deckhand aboard Galactica, and was having dinner with her parents when Brother Cavil came to find Simon.

Following an argument between her parents, Giana took Jemmy with her, leaving Simon on his own (TRS: "The Plan").