That's an interesting question. Puzo could have created a Sicilian consigliere and, with his storytelling powers, that character could have been just as interesting as Hagen. But having Hagen as a non-Sicilian may have given him more opportunity to add those storytelling details that make the novel so rewarding. I like it that Sonny took him in off the street and that Vito accepted and nurtured him--first as a son, then as consigliere. Puzo also takes advantage of plenty of opportunities to contrast Hagen's Irishness with the Sicilian surround. For example, during Connie's wedding, he makes a comment and shows that he's impressed with an expression that's curiously Italian on his Irish features, or some such. Vito tells him, "Even though you're not a Sicilian, I made you one." And he tells Michael that he puts on his "Sicilian hat" and figured out what he was planning re. the Great Massacre. Nice touches, all.