Hiro is becoming a Royal
As his servitiude to Hime continues, he's able to access his "other side" with more self-control and for longer periods. Basically this involves increased strength and agility with Red Eyes
with Hellish Pupils
. Exactly the same traits Hime herself has. Yes, other Blood Immortals get the eyes, but Hiro is going past that point.
- It's possible that becoming a Blood Immortal Servant turns one into a classic monster, and Hiro was a Million to One Chance of a mutation into the Royal Monster category.
- Or maybe he was a Royal all along; who was punished by becoming a human for some reason. The Blood Immortal bond is just breaking through that conditioning. His sister may have been a former (non-blood immortal) servant; or more sinisterly, someone left to keep an eye on things and is reporting to someone else.
- Sherwood has commented on how Hime "hasn't told Hiro everything" and that he'll "become a very strong Blood Immortal."
- Jossed. As of the series close, Hiro becomes a flame warrior, much like a blood warrior but with added flame gifts from the mature phoenix who granted him the flame of life, while also meaning he no longer needs royal blood to survive.