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...