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Hope I don't count the wrong chapter number.
Finally, several questions has been answered.
About the One Phoenix
Fighting physically is meaningless, as seen in Fuhito and Sylvia's fight. Instead, phoenix battle each others mentally. A phoenix will merge with another and then their personality will fight each others (although the one making the stage have the advantage).
If one is defeated, their will is subdued and they become like doll to the winner. This is what happened to Fuhito. While very strong in the physical world, seems like his mental power is not strong enough against that scary sister.
And the cockblock fact about the strongest blood warrior
Usually, a blood warrior can only protect their master as best as they can. The strongest one is no different. But Hiro is able to protect Hime everywhere, even places no other person can enter.
Even when Hime is trapped in Sylvia's consciousness, Hiro can break in to protect her.
Probably this is also why Hiro can kill Hime's brother with the invisible man in the Back to the Future episode even though as a visitor from the future he shouldn't be able to affect the past.
As for Reiri x Riza shipping
After Kinisky is dead for good, his land is taken by the vampire's council and Reiri's childhood friend Greta came to pick her up (cue the usual vampire-werewolves bickering when she met Riza's friends). More importantly, it's suspected that Hime will lose against Sylvia, so Greta want to take her friend out of there as soon as possible. Better go back to the usual vampire's carefree life rather than getting caught with the losing side.
Reiri refused and wanted to stay. Vampires do things their own way and not even friends can tell them what they should do. It's just their style.
Greta thought that Reiri likes Hime, but Reiri's worried about Riza being called to a questioning by werewolves council for suspicion of working as spy for the vampires.
Riza, insist on speaking the truth that even though Reiri is a vampire, they're partners who have risked their life through battles together. Whatever happened, she's not selling out her friend.