I'm gonna guess Rin "splitted" in 2? His demonic side and his human side?
It looks like a common trope in manga (Hero has to fight with his evil self), and it's like the dual of anime Ep24 (but much better)
Oh, that's right! And here's another reference.
Kato-sama AKA everything you'd NEVER expect. Really, the title of this arc should be: "Beyond Kazue Kato"
Edit: is it me or Rin's demonic heart shape looks like the vanishing key's... um, thing at the end? So the two things are actually related.
Edit 2: so stupid, I just noticed. Dressed!Rin's traits are back to human version (his ears, teeth and he got no tail), so it's really Rin's human side vs. Rin's demonic side. And in the last part, he got "half length" ears and a longer, white-ish tail (lil horns aside) so is it possible now the two sides "melted" into one?
Edit 3: since we're still in the Beyond the Snow arc, what if what's happening to Rin is related to Yukio somehow. Like, what if Yukio (anime style) got a demonic side hidden somewhere he got splitted from?! It's too soon to theorize and I'm talking nonesense. I better stop.