Anybody who teaches a skill, which coaches do, is admirable. But sport doesn’t build character. Character is built pretty much by the time you’re six or seven. Sports reveals character. Sports heightens your perceptions. Let that be enough.

Heywood Hale Broun (1918-2001) American author, sportswriter, actor

In Ames Daily Tribune (16 Jan 1974)

Broun used a number of variations of this idea. It was more famously paraphrased in James Michener, Sports in America (1976), as "Sports do not build character. They reveal it." More discussion on this quote here.