Of the four final Zerozaki novels, this was the one I was most looking forward to -- the Zerozaki series kicked off with Iori as the point of view character, and I was certainly eager to see more of her. A brief scene in the third novel involving her getting some new hands was hardly enough, and her relationship with her brother was still barely defined.
Apparently it was barely defined because he wasn't really sure what to do with it. Zerozaki Hitoshiki, like Ii-chan, is somewhat defined by his inability to relate to people around him. He's naturally quite resistant to admitting he has a sister, and Nisio Isin eventually defaults to only demonstrating the strength of their relationship in stock combat-skills-increase-when-family-is-threatened moments...which are a central part of what a Zerozaki is, and certainly work, but don't exactly resolve the central relationship in a particular satisfying manner. He also hampers himself with an extremely misjudged bit about Hitoshiki dying, and possibly not being a true Zerozaki at all. It just feels muddled; I remain unclear on whether it was even a red herring.
Fortunately, the rest of the book delivers in spades. A direct sequel to the end of the Zaregoto series, the plot involves Aikawa Jun on a mission to resolve several bits of business the main series ultimately had no time for.
Particularly Ishinagi Moeta and Yamiguchi Houko.
Bit of back story here -- according to the Zaregoto Dictionary, there were actually two versions of the third novel in the series, one of which never saw the light of day. It was intended to be a novel focusing on the residents of Ii-chan's el cheapo apartment building; several of them had been introduced in the second novel, but he added more to the cast and attempted to spin a story about them...which never got interesting. The result was that later books in the series had several peripheral characters who lived in his apartment, and where frequently mentioned, but rarely actually played an active role. Nanananami Nanami never showed up at all, and the two runaways -- fifteen year old Reaper, Ishinagi Moeta, and his sister, the thirteen year old Assassin, Yamiguchi Houko, half-siblings from two of the seven killing families -- did not play an active role until Uprooted Radical.
While both their stories are resolved in the actual novels, their backstory remained a complete mystery; how would members of these two families come to be related, and why would they have run away and living in Kyoto?
So this novel retcons a scene with Moeta hiring Aikawa Jun to help Houko resolve issues with their parents, and she winds up taking Houko and the two surviving Zerozaki to the Yamiguchi's secret island to have it out with Houko's dad, the undefeated Rikka Kajumaru. Rikka Kajumaru is a seventy-eight year old undefeated legend...who is very enthusiastic about having a child with each of the seven killing families, and quite excited to discover that a female Zerozaki still exists. His scenes with his estranged daughter provide exactly the kind of visceral resolution that the title relationship wound up lacking.
On balance, probably the most satisfying book in the series since the original, but I can't help still wanting to see more of Iori; hopefully she'll show up in the final volume as well. I would definitely like to see her meet Ii-chan.