So, what would a relationship between Kozakura & Sorawo look like? I think that's the fundamental question to figure out in order to understand what the SoraToriKoza ship looks like.

We know what it doesn't look like. It's not Accomplices - Kozakura cannot be the person going to the Otherside with Sorawo, she can't be attached for life even if they come to hate each other by their shared experiences. The fact that she can't be that is in fact the biggest detriment to that relationship.

It also can't be a Nue. I can't imagine Kozakura calmly going to the deep blue Otherside, experiencing it with Sorawo in complete and total trust, letting Sorawo change her and expose her deepest selves. Kozakura has already been changed by Sorawo's eye once, and she hated it, and she feared it, and she absolutely does not feel the same way about it that Toriko does from the getgo.

What else has the story proposed? Kozakura's self imposed role is the Adult of the group. She admonishes SoraTori for their youthful indiscretions and tries to push them to be more mature. She tries to teach them appropriate social skills and to be more future-oriented. She tries to give them opportunities to grow their relationship that they wouldn't otherwise have. This is definitely where Kozakura puts herself, though it's a weird dynamic for a romantic relationship, and one that Sorawo would definitely resent, given how much she hates seeming immature to others.

The other proposed answer is an Anchor. The person that SoraTori come home to, the thing that ties them to this side of the world. The thing they fight for, to make escaping the horrors and going home worthwhile, instead of the two losing themselves to the Otherside together. This is particularly interesting, because it's a role that is diametrically opposed to the other two kinds of relationships she can't take - Accomplices & Nue need the influence of the Otherside, because they're both about exploring and becoming a part of it. The Anchor, though, is about coming home, about maintaining relationships and attachments to humanity, about being the reason they don't become part of the Otherside

So, if Kozakura's proposed roles are Adult & Anchor... well, there is one character she is both of those things for. Kasumi. She's Kasumi's guardian, the Adult in her life making sure she's taken care of and growing & learning & gaining social skills and all that. But importantly, Kasumi is also someone who traverses the Otherside, much more often and with more ease than SoraTori do, and by taking her in, Kozakura is serving as her Anchor, as the person to return to at the end of the day. Kozakura has fulfilled both of the relationships the story has promised her by taking in Kasumi. She doesn't need them from SoraTori.

I think that statement is what actually kills this as a poly story - I don't think SoraToriKoza will happen because Kasumi resolves Kozakura's arc on her own. But, let's chase it anyway.

I would propose that the kind of relationship the three of them could have is Moms. Three Moms, living together to take care of their feral Otherside daughter. SoraTori can be Kasumi's connection to her Otherside heritage, and Kozakura her connection to humanity and this side. And they all three navigate that together, and fill Kozakura's lonely mansion with life together.