I think what they did with Hunter was very clever
So the original theme of Fionna and Cake is gender swapping, that's how every character is designed, a gender swapped version of a character from the original show, every character follows this straightforward theme, seems simple at first, then there's Hunter, a special case that fits it even better.
Hunter's character is non binary and transmasculine, meaning they were assigned female at birth, there is debate on whether Huntress wizard, Hunter's adventure time counterpart, is also trans, but right now we're going to assume she isn't, and I think this works exceptionally well because it not only follows the theme but makes a clear distinction between biological sex and gender.
Both characters were born female, but while Huntress wizard identified as a woman, Hunter did not, it shows that gender is not dictated by biology and helps the theme of simple gender swapping involve into something much more poetic, which I think is really cool.
Of course this is all assuming both Hunter is trans, which I feel is quite heavily established, and that Huntress is biologically female, I know the theories of her being trans or possibly from a species without biological sexes, but still, if this is what they were going for, I think it was beautifully done.