The CoCCCG card State Representative (U39) is a Syndicate faction character, but appears to have an Agency symbol before the title in the Steadfast position. Does this mean that I would need to have at least one Agency resource in one of my domains to play him?

that is strictly correct, as long as a character as a steadfast symbol, that means that this character might show up only if you get at least the number of steadfast icons on the character title in you whole domains.