Adventure Time: Fionna & Cake offers a primarily positive portrayal of LGBTQ+ characters and themes. Queer relationships, notably Marshall Lee/Gary Prince and Bubblegum/Marceline, are normalized, integral to the narrative, and depicted with dignity and emotional depth. The series embeds queerness naturally into its world-building, reflecting an affirming and inclusive approach without sensationalism or marginalization.

The show features Fionna, a central female character, who consistently engages in and wins close-quarters physical combat against various male adversaries. Her combat style involves both hand-to-hand techniques and melee weapon use, leading to successful outcomes in defending her world.

The series' core premise involves gender-swapped versions of established characters from the original "Adventure Time." For instance, Finn the Human is reimagined as Fionna Campbell, and Marceline the Vampire Queen as Marshall Lee, fulfilling the definition of a gender swap for multiple significant characters.