Number One
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Brother Cavil is a priest who provides counsel for Chief Tyrol (Lay Down Your Burdens, Part I) after Tyrol's maddened attack against Cally. Cavil identifies the source of Tyrol's anxiety as arising from the fear that he could be a Cylon sleeper agent.
A somewhat eccentric figure, Cavil is a surprisingly harsh critic of prayer for a priest, but claims to have been preaching "longer than you [Tyrol] have been sucking down oxygen."
It was revealed in Lay Down Your Burdens, Part II that Cavil is a Cylon, seemingly with the core purpose of delivering an important message to the fleet...
Cavil is portrayed by Quantum Leap's Dean Stockwell.