Adventure Time's only human character is a white guy

TV show Adventure Time pretty much has gender diversity, but doesn't have racial diversity. Its only human character is a white guy, but its non-human characters have a reasonable male-to-female ratio, as well as allusions to transgender and gay characters, and interracial relationships.

Representation of a variety of characters is important for a bunch of reasons:

1. Stories about a group of people humanize those groups of people for voters and policymakers. When a dominant group holds power over a minority group, and the dominant groups thinks negatively of them, the dominant group will set up legal policies harmful to that minority group. For example, Voter ID laws disproportionately prevent minority voters from voting. This would not happen if dominant-group policymakers actually cared about those minorities.
2. Role models are an important part of human development. If I see someone on TV, who looks like me, flying to the moon, I feel like I can fly to the moon, too. If, in my 23 years of development, I've barely seen anyone on TV who looks like me at all, and the only ones why do are criminals or background characters, that has an impact on my perception of whether I can achieve the same kind of success as the guy on TV who flew to the moon. So I won't even try. This phenomenon has been empirically proven.

To positively portray a woman or a person of color on TV, give them their own personality, goals, motivation, agency, capability, and everything else that you've already automatically given to the white-guy main-character. You might have to learn some film school and some feminism.

Then rewrite them so that they're not adhering to the stereotypes that you inevitably wrote into their character on the first draft.