Warning: spoilers follow.
Milo, like most characters in Catch-22, is the subject of his own chapter (chapter 24) but is mentioned much earler. He is one of the main characters in the novel. His most interesting attribute is his absurd logic in the operations of his enterprise.
Milo's enterprise becomes known as "M & M Enterprises", with the two M's standing for his initials. Milo travels across the world, though mainly the Mediterranean, trying to buy and sell goods at a profit. Everyone in the camp has a "share", which Milo uses to defend his actions, stating what is good for the company is good for all. Milo even begins contracting missions for the Germans, fighting on both sides during one battle, and bombing his own US camp.
In typical Catch-22 satirical fashion, Milo's business is incredibly profitable, with the exception of his attempts to sell chocolate-covered cotton.
Milo is a friend of the main character, Yossarian, and tends to trust Yossarian more than he trusts anyone.