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Jacob Cantrell was Sagittaron's representative to the Quorum of Twelve ("The Ties That Bind", "Escape Velocity", "Guess What's Coming to Dinner?") prior to his assassination along with the other Quorum members during the Gaeta-Zarek mutiny (Blood on the Scales).
When Roslin goes missing after a Cylon baseship she visits unexpectedly jumps away, the Quorum squabbles over her successor. Cantrell pointedly asks Lee Adama whether Admiral Adama would support Tom Zarek or the government if he were to become president. After some hesitation Adama replies that he wouldn't, leading to more shouting (Sine Qua Non).
Following the discovery and evacuation of Earth, Roslin considers Cantrell the key to persuading the Quorum in favor of an alliance with the rebel Cylons, but he is one of the strongest voices against the plan (A Disquiet Follows My Soul).
Later, after Lieutenant Felix Gaeta and Zarek stage a mutiny aboard Galactica and Zarek turns to the Quorum for support (minus Lee Adama who was at large aboard Galactica), the Quorum rebuffs him and Cantrell orders him to leave, pointedly addressing Zarek as the Vice President. Zarek calmly leaves the room and orders the marines outside the door to execute all Quorum members present (Blood on the Scales).