Well, first we have to look at who the main characters are -- Gatsby, Nick, Daisy, and Tom. Since a minor character would most likely be described as one who has an effect on the actions or emotions of the main characters then we can say that the three most important minor characters are Jordan Baker and Myrtle & George Wilson. Jordan has a major impact on Nick, particularly in terms of his feelings and the information that he acquires throughout the novel. If it had not been for Jordan, then Nick would not have known much of anything about Gatsby and Daisy's past. Myrtle is obviously important because of the role that she plays as Tom's mistress, and it was her death that indirectly got Gatsby killed by her husband George. The character of Owl Eyes is also a minor character because he represents the same idea as the billboard in that he sees everything that is happening -- which is recognized the last time we see this charcter at Gatsby's funeral. Other minor characters include Klipspringer, the McKees, Katherine, and Michaleis.