Princess Resurrection/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. We've seen other Blood Immortals; but they haven't shown the same multiplication of strength that Hiro has.
 * 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."

The Paperbag man of Akasabi Village survived being killed by Hime.
Severely weakened, he had to take lesser souls to survive, but he was permanently destroyed by Maka. She refers to him as Sonson J, the killer of Emerald Lake."

Sawawa has experience serving monsters
That's the real reason of her Weirdness Censor...she is just used to a lot of those things. In fact, she perfectly aware 90% the people in the mansion aren't humans, it's just that nobody asked her if she knows that...she is probable unaware of the "Royal Family" and the whole battle thing, though.