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Valance is an assassin who might have connections with Tom Zarek. All indications lead to his professionalism due to his expertise in sneaking a ceramic gun past the X-ray scanners. It is likely that his intended target is President Roslin, seeing as all the events she is to attend are circled in Valance's briefcase (which itself is constructed with a compartment that conceals the gun and is impervious to X-rays).
He is captured shortly after it is discovered that he has a gun (a stroke of luck, seeing that a bar room brawl causes his briefcase to be knocked over, and the gun to fall out). Captain Lee Adama and Lieutenant Kara Thrace interrogate him with less than conventional methods, which gets nowhere. They hope that Zarek is involved, in order to discredit him from obtaining the Vice Presidency of the Colonies.
He is killed by an unknown party during his incarceration aboard the Cloud Nine, the assassin apparently sneaking into the room via the air duct.
Zarek comments to Roslin that he does not know who might have killed Valance (Colonial Day).