John Cavil, a.k.a. Number One, is a humanoid Cylon model who appears as a highly rational, eccentric man in his late-sixties. Cavils have a heavy sarcastic demeanor and a sense of humor. They take neither religion nor death seriously, and are the only atheist Cylon model. A Cavil is first seen aboard the Galactica assuming the role of a priest, offering spiritual guidance, and going by the name "Brother Cavil".
One of the unique characteristics of the Cavils is the sadistic, cruel and Machiavellian streak in their personalities. Cavils openly mock their fellow Cylons every time they attempt contact with humans, even when it is absolutely necessary.
Viewing death as a nuisance, Cavils, however, are extremely displeased with pain (placing the blame on the Final Five). This view fuels the idea that Cavils wanted death to the humans to be as quick as possible, but were unprepared for the aftermath. Their attitude towards mankind's mortality, rather than irrational hatred, derives from their overall lack of knowledge about absolute death and the consequences that it carries. As revealed onBattlestar Galactica: The Plan, the Cavil that was stationed in Caprica learns from Samuel Anders the motives for the resistance; that humans do not leave anyone behind, even their dead. Respecting their resilience, the Caprica Cavil agrees with Caprica Six and Sharon Valerii (Boomer) to abandon the Colonies. Disappointed with the other Cylon models after failing to accomplish anything through boycott and sabotage, the Galactica Cavil fails to learn any lesson. As they are discovered and airlocked from Galactica, Galactica Cavil swears to "box" Caprica Cavil for his insubordination and is indicated to be the same Cavil that is the main enemy of season four given his plans to destroy humanity once and for all once he resurrects.
It is later implied in the series that the Cavils, akin to their creators, lack superhuman strength, and are actually the least likely to defend themselves. It is also implied that the Cavil who led the Civil War, and who killed himself in the CIC when he realized he was losing and had no chance of winning, is the resurrected Galactica Cavil. Like the Simons and Number Threes, and in contrast to Leobens, Dorals, Number Sixes, and Number Eights, Cavils appear to be relatively uncommon and never appear in large groups.
The Cavils' superior awareness of all situations (attributable to their being the only human model Cylon with full knowledge of their origin) and uncanny manipulative traits make their series the de facto leaders of the Cylons. However, their vast superiority and manipulations over their fellow Cylons goes beyond comprehension, as they manage to annihilate the Twelve Colonies and repeatedly attack the survivors without hurting the Final Five, and ensure their survival only to make them 'the ones who suffer the most', and basically carrying on with the annihilation of billions only to teach their creators a lesson.
A comment made by Sharon "Boomer" Valerii indicates the Cavils are against the idea of the Cylons reproducing themselves biologically, which makes sense given their distaste for human traits as well as lack of faith in the Cylon god, whose commandments include, "Be fruitful". Ellen Tigh also mentions that the idea that the only hope for the future of the Cylon race might be 'messy biological reproduction' is too much for Cavil.
Cavil is very insistent the Cylons not seek out or discuss the Final Five models. He also actively tries to prevent D'Anna from learning their identities, even threatening her at gunpoint in "Rapture". After she sees their faces, he boxes her before she can tell anyone, while claiming support from the other Cylons. The reason is he knows the identities of the Final Five and doesn't want them exposed (he’s waiting until they've learned their "lesson.")