Ray Kinsella is the main character. He is impractical; he is an idealist, a dreamer. He was close to his father and one of the strong bonds between them was based on baseball. Among his wife and daughter, Ray lists baseball as one of the loves of his life. Using baseball as a conduit, Ray is able to live out his dreams and the dreams of others. He is also able to rekindle relationships with his father and brother. To an outsider, Ray would appear to be so unpractical as to be crazy. Salinger mentions this multiple times when they first meet. But in the end, it is Ray's idealism and determination that makes the impossible possible.
If there is a protagonist (or "bad guy") in this story, it is Mark, Annie's brother. Mark is businessman whose main motivation is profit and progress. He wants to buy Ray's farm and modernize it. Ray opposes this in principle and when the ballpark is built he has even more reason to oppose Mark's intentions. It isn't until the end that Mark begins to see the potential of the field.
Shoeless Joe Jackson was a professional baseball player (in the book and in real life) who was banned (many say unfairly so) from baseball in 1921 for throwing the 1919 World Series. Ray initially builds the field for Joe Jackson to come back and play. This is the first instance where Ray helps someone else realize their dreams. Joe is/was a hero to Ray and his father. Joe also exemplifies Ray's idealist/purist spirit, claiming that he plays for the love of the game, and that:
I'd have played for food money. I'd have played free and worked for food.