"Number Six" is a humanoid Cylon who plays several key roles in Cylon society. She is the first humanoid Cylon that viewers witness in the re-imagined Battlestar Galactica. Stunningly beautiful, sensual, and highly religious, this highly driven model is marked by a ruthlessness against all who stand in her way.

To date, more unique versions of the Six model have been witnessed than any other humanoid Cylon. Besides the use of numerous aliases, Six is also the only Cylon model known to have varied appearances and personalities: while most often seen as a platinum blonde, Six has also been seen with honey blonde and black hair in varying styles, and displaying personalities ranging from overtly sensual to wide-eyed and innocent, and from blindly supporting the attack on the Colonies to desiring peaceful coexistence. This differs from the other known models who generally display the same personality traits (Sharon Valerii and Sharon Agathon being exceptions), use the same name, and have fewer cosmetic differences in appearance.