Champion
The person inside the customer who wants your project to succeed and spends their own credibility defending it.
A champion is the internal advocate who gets you access, defends the budget in rooms you are not in, and tells you what is really happening politically. Deployments without one stall regardless of how good the engineering is, because someone has to argue for the project when it is inconvenient.
Worked example: the highest-leverage hour of an engagement is frequently an unscheduled lunch with the champion, where you learn that a VP is sceptical and why — which reshapes the next demo entirely. Gotcha: a champion has finite internal credit and spends it on you; going silent for three weeks or handing them nothing to show is how you burn it.