The phrase "biggest cheerleader" refers to someone who is extremely supportive and encouraging of someone else's efforts and achievements. Full definition
March 30, 2018 — As presidential candidate, Donald Trump declared the Common Core a «total disaster» while promising to be the «nation's biggest cheerleader for school choice.» (educationnext.org)
And leading the list of governing board members is Carl McCluster, who also happens to be one of Perry's biggest cheerleaders in Bridgeport. (jonathanpelto.com)
But even the industry's biggest cheerleaders acknowledge the risk posed by the wave of store closings. (fortune.com)