I'm still a skeptic on the "they're going to use up the remaining classpects" theory, as I have been since the epilogues dropped, so take this with a grain of salt, but I could very easily see Yiffy as either a breath or void hero. Breath due to the strong themes of lack of freedom and control over her own life and void due to her being hidden right up until jade announces she's been taken political prisoner, followed by only showing silhouettes and vague conversations about her until she actually debuted. Even now the most we really know about her that's come from her is that she's gained some respect for Tavvy and she's done dealing with the adults in candy. I feel void definitely is a weaker guess than breath for her but I do still feel its probably a better guess than rage, simply because I feel that guess has mainly been bolstered by the aforementioned "remaining classpects" theory and her growling at Jane/disrespecting Gamzee's funeral/saying "fuck you" to the candy adults also people hoping she dislikes her own name as much as they do, and I have a hard time seeing her actions through a lens of tearing down lies and systems upheld by them. Confusion and doubt brought into the story may be a better argument for her being ragebound but I feel people experience that similarly when they're first introduced to equius and he isn't ragebound either.

anyways tl;dr I think remaining classpects theory isn't that solid and yiffy's probably a breath hero.