After 91 chapters I think I've actually reached the end of my rope for the series. The characters are for the most part cookie cutter molds that have nothing to mold. Except for maybe most of the vampires and Shinoa.
Yuuichiro has as much depth to him as a toilet bowl. His entire raison d'être is to protect his family and leaving aside whether or not that is good (I think it's admirable and a great quality to have) the way it's developed and shown is just painful. For one, Mikaela and Makoto have good arguments why they shouldn't do what they're doing (i.e. trust/follow Guren, Felid, Crowley, etc.) but this is waved aside because (a) the character in question is not entirely bad and (b) they must have had their reasons.
The problem with this sort of reasoning is that (a) fails to hold up as a defense of a character prima facie because no one is entirely something (unless you're some sort of reductionist). So Yuuichiro can't coherently defend the characters he does without considering the arguments Mikaela and Makoto present but he doesn't do that (but to be fair Yuuichiro is presented as a sort of an idiot with a heart of gold but idiocy has its limits and stretches our willingness to cheer for him; even Luffy and Goku knew where to draw the line). (b) is just plain bad. Again prima facie there could be said there is always a reason for something but the mere existence of a reason does not necessitate a character as good/just.
So we have it according to Yuuichiro's logic, Guren (or any of Yuuichiro's comrades) could genocide an entire race and as long as it is for a reason then it is okay. I'm not sure about you but that isn't a character I'd like to root for (unless the author makes him enjoyable to see onscreen though Kagami Takaya doesn't do that here). So we have it that Yuuichiro's modus operandi is that the "ends justify the means." More specifically, the protection of his comrades come before anything else which is typical shounen manga values but that's about it. There's nothing really else going on. All actions taken by him are to that end and that's what I mean by Kagami Takaya mistaking a character type (the standard shounen manga hero) for a character.
Shinoa is probably my favorite character because although she fits in the kuudere mold she doesn't fit it to a tee. She's playful, smug, and a tad mysterious; in other words its fun to see her on screen. The only aspect I wish was better with her is the romantic subplot between her and Yuuichirou.
The setting is interesting, but the thing is that its cluttered when the story starts. There are vampires, angels, monsters and humans. You don't get a clear sense of who is fighting who and what caused what. We have it that (a) the world is ruined, (b) vampires and humans are fighting, and (c) monsters are wrecking anything and everyone they see. Well okay; I can take (a) at face value but what about (b) and (c)? What is their relation? Is (c) related to the vampires or humans? Who do we root for in (b)?
In stories like these its good to clearly separate the side we should root for because it helps the story move along easier and tightens the narrative focus.Instead we get a cluttered and episodic focus that makes it hard who to root for until, maybe thirty or so chapters later (or at least until Krul, Felid, and Guren's machinations are partly revealed). It only gets worse once they introduce "God" as some sort of antagonist for the patriarch of the vampires. It screams of triteness, of a typical JRPG story that fails to leave me with bated breath.
And now for the narrative. This is less of an issue than the first but still a major one. Kagami Takaya clearly set up a complicated narrative for Seraph of the End but he split it up between several spin-off series that ended up taking away from the main series in terms of narrative build-up. In other words, the build-up isn't in Seraph of the End but in Seraph of the End's spin-offs.
Now I can see why he would want to do that: (1) he can easily reduce the amount of time the series is running in a magazine by offloading critical plotlines to other side-stories and (2) potentially increase revenue. (2) is trivially easy why he would do so but (1) makes it a bit harder to justify because it impacts the main series which is the series most read and the series the anime aims to advertise (i.e. the series entry point). So you get things like Mikaela's time spent between the start of the series and the time he met Yuuichirou cut out out where in fact it could have helped establish his character and flesh out the world.
Same thing with Guren and his backstory and even the vampires. Hell, Shinoa is revealed to have some sort of demon but that isn't revealed until 50-60 chapters in. I mean we know she has some sort of relationship with her sister (who is revealed to be Guren's lover and reason for him being so fucked up) but this isn't really elaborated. Dropping plot points without foreshadowing them (Saito isn't really foreshadowed, just sort of dropped onto us) makes it hard to feel the weight of the twists Kagami Takaya employs.
Overall, it's hard to like Seraph of the End on the grounds that it's characters, especially its protagonist and deuteragonist, are so poorly characterized, and its narrative so splintered. In all honesty the art is probably one of the reasons why I even bothered with the series.