Champion
This is the individual in your customer's organization who does internal advocacy for the sustained, continued and/or increased use of your product. They believe in the product's promise and speak on behalf of the relationship at internal meetings, support renewal, and drive expansion discussions.
They're in most cases the day-to-day key contact person(s).
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THE TRUTH: Your product champion is only as influential as the power they wield. A promoted champion is more valuable today than they were yesterday. The reverse holds true. A champion intending to exit the customer organization is a renewal risk. The strength of the champion relationship is measured by organizational reach, not warmth