And also bunkaisae

First off, IsaNess and RyuRin are literally together because both Ness and Shidou crave for someone close enough to the person they couldn't get (Kaiser and Sae). So they settle for Isagi and Rin. It's toxic and they're still pining and yearning and they really, really shouldn't have found a replacement or stand-in but well, feelings are illogical!

On the other hand, Isagi and Rin are intimately aware of what Ness and Shidou are doing and... let them do it. They let them get away with it because they both wanted, for once, to be chosen. They wanted to bathe in that high that they were the ones that got picked over whatever monstrosity Kaiser and Sae are (do they love Ness and Shidou? Maybe, maybe not. Maybe they're in love with the idea of being loved).

This makes a cesspool of toxicity because Ness and Shidou don't love Isagi and Rin, they just want a replacement, and Isagi and Rin don't even love them back as much because they only crave the warmth of being loved.

Enter Kaiser and Sae back to the picture.

The second they start reaching back out to Ness and Shidou, Isagi and Rin panic. They panic HARD. They panic so much that they're about to lose the one thing that soothed their empty souls and teamed up to get Kaiser and Sae together. Will this hurt Ness and Shidou irrevocably? Yes. Will Isagi and Rin still do it anyway to serve their selfish need to be loved? Yes.

So, they do that in the background and Ness and Shidou's heart breaks part two. Because now, they can't even reach Kaiser and Sae even MORE. But it's PERMANENT and not just a distance or gap thing anymore. They can't have them because they're literally happier together and that hurts.

So, they run back to the waiting arms of Isagi and Rin. And it's toxic and Isagi and Rin exchange a look over the shoulders of Ness and Shidou. But. They got what they wanted.