Is there a word for someone who makes others better?
A person who motivates others could be a word you're looking for.
Example:
Steve Nash was a motivator.
If they're facilitating teamwork or getting others to combine their strengths, you could call them a catalyst. For example:
You take anybody’s star player or players [out] of the lineup, it’s a big hole to fill. The one thing that Connor really did well was energize our group. He was a catalyst and also promoted some real strong play from others. (source)
If they're elevating the performance of others by setting a good example, you could call them an inspiration.