Ben Loman is Willy's older, late brother. He appears the most in the visions that ocasionally hit Willy Loman when he enters into periods of severe stress, in which he seems to lose his sense of reality.
From what we learn about Ben in the play, he actually succeeds at becoming rich and finding fortune, since he is a man of action, not words, unlike Willy. He goes to Alaska in search of Willy's father (who abandons them) and instead found fortune off the diamond mines in Africa. We know that Ben is Willy's foil in that he is just like Willy in terms of personality, but contrasts tremendously with Willy in terms of risk-taking. He also belittles Willy. He is who Willy could have become if he had not elected to follow "the wrong dream".
We also know that Ben has the same weaknesses as Willy: Is conceited, encourages bad behavior, and is haughty without control. However, success gives him the upper-hand in life.
Basically, Ben is actually a minor character in the play, for he only makes brief appearances. However, his influence in Willy Loman's life is what, in part, leads Willy to make all of his life's choices.