First, it seems to revolve around a brother-sister bond, but once you get past a few chapters it becomes pretty clear that Nezuko is no different from an animal mascot because she barely talks, only there to appear cute and fight. So I am not very disappointed when the story shafts a brilliant, underused concept in favor of a more classic battle shounen trope of the protagonist having more important bonds with other male characters.
Okay, now that I have actually written that out, that's actually pretty depressing and disappointing, but not enough to take you out of the story because the story doesn't center on any themes whatsoever. It's straightforward and honest, just like the protagonist. Speaking of him, I love him, because he is so sweet, and there are plenty of fine, non verbal details that add substance to what would other wise be a typical goody-two-shoes personality. He's no deeper than that, especially since the story doesn't connect to any themes, but that's okay. Not everything has to be Fate/stay night.
As always, I often have my criticisms of the way female characters are portrayed, such as with Nezuko and the unnecessary, weakly-excused design of one of the female Pillars. But I do praise the other female Pillar, who wears the same battle outfit as any other male fighter, has important relationships with female characters, and whose friendship with another male character isn't romantic (and, there's nothing wrong with romance, but it's annoying when female characters always have to be romantically involved with a male character when the male characters don't get that same treatment).
The last thing I have to say about the female characters is that there really isn't much to say about them, and in the case of KNY it actually doesn't bother me, because none of the male characters are particularly well-developed or go through an interesting character arc either. That part is refreshing, because the "homoeroticism" found so often in battle shounen often fuels the misogyny by developing male characters but not female ones.
Anyway, maybe other people don't think the same, but I thought it was interesting how romances are implied in this manga. There are a few sweet moments that hint at things and at the end you see the result of all that in a timeskip. There are no big confessions of love, so it all feels like it would naturally happen once the events move past endgame.
Finally, I like how short this manga is, but maybe it's a little too short, considering the importance of the last arc and how many important character moments are stuffed in there. But it's not disappointing or immersion breaking.
I think this manga is something I'd look back on fondly for its simplicity and aesthetics.
