Character Role Analysis
The largely fictional part of Slaughterhouse-Five starts with the line: "Listen: Billy Pilgrim has come unstuck in time" (2.1.1).
And from there on out, Billy's story dominates the novel: the narrative of his wartime troubles, his witnessing of the Dresden massacre, his return to the United States, and his decades-long struggle to deal with the aftermath of World War II provide the primary material of the book.
While Billy is not the world's most forceful character, and we may not be left with a strong sense of who he really is as a person (for more, check out his "Character Analysis"), he is certainly the central focus of Slaughterhouse-Five.
The Narrator (Kurt Vonnegut)
As we explain in our "Symbolism, Imagery, Allegory" section, Slaughterhouse-Five constantly draws attention to the fact that it is a novel, and that the narrator is using the characters to tell the story of his own experience in Dresden.
Given all the biographical detail and reflections on how we, as human beings, can cope with pain and death, we have to cite the narrator—and the author himself—as the alternate main character of the book. Slaughterhouse-Five is like a puppet-show where we can see both the puppet (Billy Pilgrim) and the puppeteer (Kurt Vonnegut). If Billy is just a puppet, then that makes Kurt Vonnegut our protagonist.