Monster Princess/WMG

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."