The series introduces Hunter, a canonically transmasculine character with mastectomy scars and he/they pronouns, voiced by non-binary actor Vico Ortiz, who becomes a focal point of both fan celebration and backlash, including accusations of misgendering in captions. Queer relationships like Gary (voiced by gay actor Harvey Guillén) and Marshall Lee (depicted with intentional dark skin for diversity) are highlighted, alongside Bubbline crumbs and implications of Huntress Wizard's trans identity in her genderbent origins.

Themes of self-discovery, identity suppression (e.g., Cake feeling reduced to a 'freak show'), and adulthood struggles intersect with social justice undertones, such as consent discussions and community building over individualism. While emerging organically from the creative team per showrunner Adam Muto, these elements drive plot points like Fionna's jealousy and hero complex amid queer-centric narratives, leading to divided reception: progressive fans praise the casual queerness in parallel universes, while significant audience backlash from 'right-wing grifters' and YouTube critics labels it 'too woke,' citing trans rep and 'punishing' m/f shippers as prioritizing message over adventure, with titles like 'Why Fans HATE Fionna & Cake' amplifying the controversy.