After AlternaWelt: Blue Exorcist Gaiden second trailer dropped I decided to make Rin a “furry from” and here’s the headcanons I came up with for it
Imagine finally getting control over your flames and not flame up when you sneeze. But instead now your head splits in two literally and you become some sort of beast. Then you told it’s demonic puberty for Nephilims or some shit and he’s a late bloomer because his demonic nature is trying to catch up with the rest of him after being sealed for 15 years.
This transformation is tied to his emotional state at the begging, hard to control and he might need time to “cool off” so he can turn back to normal. This is actually like an “ugly duckling” state of his beast form where he didn’t fully transform and looks like this. And both his human and demon sides hate being like this and he really doesn’t want too many people knowing he can be like this. (They’ll know anyway. Especially Lightning)
Sometimes intense emotions can cause Rin to desynchronise within himself and his demon and human natures take over different heads (while usually when in sync he can even speak in unison or one of the heads can continue/finish sentences after another). Take a wild guess who is who.