Akihito is just like any full-blooded youmu, and taking out his eyes would kill himEpisode 1 introduces Akihito's immortality as if it's unique, but in episode 2, we discover that youmu apparently can regenerate indefinitely unless their eyes are destroyed. The same rules likely apply to Akihito.
- Episode 4 provides information to infer that this may not be true. Ayaka states that Hiromi's method to "eliminate" him would amount to just negating a physical plane for his body to exist, but explicitly says it would not kill him. Meaning he'd still exist as an intangible being, unable to directly interact in the mortal plane.
The scar on Hiroomi's back is from Akihito.It happened during some previous instance of Akihito's youmu side taking over.
Theories on Akihito's dad.There's gonna be some sort of huge revelation involving the dude. Place your bets!
- His dad could be Fujima as the two haven't met yet to prove other wise and Fujima seems to be some what affiliated with youmu.
- Fujima passed Akihito when leaving the Nase household in episode 5, so they did meet.
The old man in the rainbow cave is a youmu possessing the Nase family.Who is the mysterious subterranean elder giving orders to the Nase children?
- The Nase family, or whoever heads it, is possessed by or beholden to some sort of ancient youmu. Izumi did say that Hiroomi would understand her someday.
Akihito is not immortal because Beyond the Boundary is possessing him. It is possessing him because he is immortal.Despite what was said in Ep 11, he did not take any sort of damage that would test him after that. His coma was not because he was no longer immortal, having Beyond the Boundary ripped out would have killed him otherwise. The trauma (plus the Calm) meant it took him that long to recover.
The Kuriyama Mirai we saw inside the Kyoukai no Kanata was not the real Kuriyama Mirai, but an image of her created by the real one.Recall that before she actually fought dreamshade!Akihito that she took her glasses off. Recall then that when you see her again inside the Kyoukai no Kanata that she's wearing glasses, the glasses she saw inside Akihito's Megane magazine and the ones he wanted to get her on her birthday. When we see her breaking apart, it's because she's no longer holding the Kyoukai no Kanata in her body and thus her image created by both her and the Kyoukai no Kanata are breaking apart. The reason we see her again at the end of episode 12 without glasses is because she has recovered on her own.
- Following this, the significance of her ring disappearing at the end of episode 12. The reason it disappeared from Akihito's hand is because it was part of image!Kuriyama. It is possible that she managed to keep that little part of her shade-image for Akihito while she recovered. With her fully healed, she no longer needed to keep it active. Kyoukai no Kanata, and by extension, Akihito, knew this and felt that Kuriyama was close, as he could sense her by her blood, which was still both in him and inside Kyoukai no Kanata.
- It is perhaps possible that she was somehow able to leave a portion of her blood(as corrosive as it's normally supposed to be) within the glasses that she left behind, and her blood is potent enough to regenerate From a Single Cell.