A "powerful ally" refers to someone or something that can provide strong support, help, or assistance in achieving a goal or overcoming a problem. They have the ability to greatly influence a situation or outcome in a positive way. Full definition
If the time is taken to level him up he will swiftly become the most powerful ally in the game. (dragon-quest.org)
Maybe the most powerful ally of optimism is denial. (si.com)
The campaign emphasizes that women can be powerful allies for each other at work — and are uniquely qualified to do so because we experience many of the same challenges. (thefashionspot.com)