spoilers for AoT season 2 and kind of 4(?)
she willingly signed up to go back to marley and be eaten. Definitely a vore fetishist in my opinion
It was a pretty dumb decision in my opinion as well. Her fetish for vore exceeded her love for historia, which is quite a lot considering ymir wanted to marry historia. This theory might actually be bigger than we realized.
So in the last few episodes of season 2, we know that ymir was struggling a lot with whether to return to marley or stay with historia, or bring historia with her to marley. It's possible that those three scenarios were different in her mind.
Return to marley dying by her vore fetish, bring historia to watch ymir being vored for humiliation purposes, or the decision she outright rejected: staying with historia and no vore. Perhaps she had the option of staying in paradis with historia to vore her over and over, but that gets shut down after realizing her stimulation only comes from being vored.
In marley when she was about to be vored, but it seemed like she was regretting it. This would lead me to believe she either likes doing the voring, or is just really conflicted and can't figure out what kind of vore she wants to be a part of.
In short Ymir can't make up her mind about the specifics in her vore fetish. Very interesting subplot that I think is really under-conversed within the aot community.