But above all, the series committed to building out its characters. Its roster runs into the hundreds; guest voices have included Marc Maron as a talking squirrel and Maria Bamford as a slime princess and a piñata. “Adventure Time” shares with “Orange Is the New Black” and “The Simpsons” the belief that any character, however minor, should be well-imagined enough to be the star of their own story.

This is especially true of the show’s villains, who tend to have sympathetic, even tragic back stories. The Ice King, one of the series’s first adversaries, was once a kindly antiquarian, driven insane by a magic crown he used to try to protect 7-year-old Marceline (the aforementioned vampire) amid the ruins of the Mushroom War.

Evil, in “Adventure Time,” doesn’t just exist. It comes from somewhere, often from good people and good intentions. The Earl of Lemongrab (a shrieking, citrus-headed cannibal) was a Princess Bubblegum experiment gone wrong. In one of the most weirdly affecting episodes, “You Made Me,” he confronts her (“I am alone! And you made me like this!”), and she realizes that she has a responsibility to help him rather than simply destroy him.

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The princess also created the show’s final big bad, her uncle Gumbald, in an effort to give herself a family. Families, especially absent or estranged ones, are a big theme of “Adventure Time.” In one extended mini-series, Finn sails off to finally discover the secrets of humankind’s fate and his missing parents. In another story line, Marceline tries to patch things up with her own dad, an irresponsible demon-king addicted to sucking souls.

This is heavy stuff for a young audience, which is to say, it’s perfect stuff for a young audience. “Adventure Time” exists in a kind of liminal zone between the poptimistic thrills of “The Powerpuff Girls” and the phantasmagoria of late-night Adult Swim.