The main themes of Jason and Marceline involve entering adolescence. Jason finds himself extremely interested in the physical changes his female classmates are undergoing. He also is interested in and puzzled by the physical changes and emotional changes he is undergoing, especially his awakening sexual feelings. Another theme involves judging people on the basis of appearance alone. Jewel Fiorito, for example, develops physically so much during the summer that the boys do not recognize her when she returns to the ninth grade. They decide that since she is so well developed, she must be sexually active. When on Halloween Jewel wears a belly dancer's costume and the boys discover that she really can belly dance, they are even surer that she is sexually active. Jason is surprised to discover her dating one of his best friends, Peter Kim, a Korean American. When Peter fights Mike Vesto, an older boy in the ninth grade, Marceline tells Jason that Peter is upset because Vesto was telling lies about his sexual experiences with Jewel. Marceline further tells Jason that she did not need Peter to tell her how nice Jewel is.
Through most of the book, Jason feels that he is engaged in a kind of sexual race with Richie Bell, another of his best friends. When Richie and Cricket Dupree begin necking in the movies, Jason feels that he and Marceline better start doing the same. No matter what happens between him and Marceline, he feels pressured to do more. He compares "this sex stuff" to football in which one team has to keep scoring or the other team will catch up. He also feels that what one does with a girl is only the first part; "the other part is what you say to the guys afterward. In fact, I was starting to think it was the most important part." Toward the end of the novel, Jason seems to be learning that sex is not like a football game.
Important to the book is Jason's interaction with a seventh grade boy who on the first day of school starts talking to Jason. The boy wears a belt with beads on it that form a pattern that looks like Rudolph the Red-Nosed Reindeer, so the ninth grade boys call him Rudy. After Marceline stops seeing Jason, Jason starts picking on Rudy, who thought Jason was his friend. Several other ninth grade boys also begin picking on Rudy. For some reason Jason does not understand, possibly because Rudy enjoys the attention, Rudy laughs when the boys pick on him and seems to enjoy it. He never shows any fear—no matter how vicious or physical the teasing gets. One day in the cafeteria, however, one of the ninth graders, Finney, begins stealing Rudy's chicken nuggets. Apparently thinking Finney will eat them all, Rudy pops one into his own mouth. Finney hits Rudy hard on the back. Rudy's glasses fall off, and Jason sees that Rudy is really scared. Jason rushes over to Rudy and uses the Heimlich maneuver to dislodge the chicken that has stuck in Rudy's throat, saving Rudy's life. Rudy thanks Jason, hugs him, and begins to smile and cry. After some hesitation, Jason hugs Rudy back. As Jason comforts Rudy, Jason notices that Marceline is smiling at him.
Jason and Marceline is also about step-families. Jason's stepfather, Ham, is very supportive of Jason and Jason's younger brother and sister. Although Jason's relationship with his younger sister, Mary, is terrible—he usually speaks of her as "cootyhead"—his relationship with his mother and stepfather is basically good, as is Marceline's relationship with her parents.
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is also about step-families. Jason's stepfather, Ham, is very supportive of Jason and Jason's younger brother and sister. Although Jason's relationship with his younger sister, Mary, is terrible—he usually speaks of her as "cootyhead"—his relationship with his mother and stepfather is basically good, as is Marceline's relationship with her parents.
The relationship between Jason and Marceline is also treated with sympathy. Marceline realizes that she is more concerned about Jason than Jason is about her when she notices that Jason never asks her about how she did in a competition to become part of the District Band. When she points out this omission to Jason, he realizes that it is too late to ask her now. Instead, he says, "I love you." It is after this episode that Marceline stops seeing Jason until after he saves Rudy's life. By then, however, Jason has learned to be much more considerate of Marceline.