Mary O’Hare is the nonfictional wife of Kurt Vonnegut’s friend and fellow soldier Bernard O’Hare. Mary appears in the framing device at the beginning of the novel, in which Vonnegut explains how Slaughterhouse-Five was born. Mary is the inspiration behind the novel’s frequent symbology relating to children and the baby Jesus, as well as the novel’s alternative title, “The Children’s Crusade.”

When Vonnegut meets Mary during his visit to Bernard’s house, he feels she dislikes him and that he may have offended her in some way, but Mary soon reveals that she’s suspicious that Vonnegut will romanticize his and Bernard’s experience in WWII. Mary feels that the romanticization of war is psychologically damaging, and she worries that Vonnegut’s novel, like much of American media revolving around WWII, will make the war seem adventurous or exciting. She points out that Vonnegut and Bernard were practically children when they fought in WWII, a profoundly tragic observation considering the unimaginable traumas and horrors that Vonnegut and Bernard were exposed to during their time overseas. Mary doesn’t want any other children to have to go through the same experience, and she believes that romanticized war stories will influence other children, like her own, to support war and even engage in one themselves. Mary’s words make a profound impact on Vonnegut, to the extent that he dedicates the book to her and calls it “The Children’s Crusade” to emphasize the tragedy of WWII.