This mouthful is named after an Ethiopian Nationalist, and he hangs out near Milton's bar saying things like "They don't give loans to black folks" (3.1.61) (perhaps he was an inspiration for Kanye?), and although he sounds crazy, he basically speaks the truth. Unfortunately, any cred he may have goes up in flames when he torches Lefty's bar because of long-simmering racial tensions.
Despite appearing on no more than three pages of the novel, Marius is an important figure in the Stephanides family epic (with a name like that, he'd have to be). Think about it: Without him, there would be no insurance settlement; without the insurance settlement, there would be no Middlesex (the house, not the novel... although without the house would we have a novel?). Just how much did he change Cal's life?