OK, I'll amend my terminology - the captain is the final arbiter subject to company rules and appropriate laws.
The point I was trying to make relates to an incident I was told about once - Captain operating from a SE European airport approached by staff in uniform and asked for the jump seat home. Captain agrees and takes staff member to check in. Just before departure car arrives at steps and unknown bounds up to the door and despatch staff say "here is your cockpit passenger". Captain refused to fly until he got the "correct" passenger. Captain was subject to pressure from local management to take "wrong" passenger but stood his ground - good on him I say.