Owari no Seraph (OnS) troubles me. I love chuuni stories like these but something about OnS makes me reel and roll my eyes. Perhaps its the fact that the characters are bland, trite, and incoherent. Perhaps its the cluttered setting. Or perhaps its the fact that the narrative twists have little to no build-up (though this is partly due to Kagami Takaya spreading his story over multiple novels). I'll structure this review into 3 parts: (1) characters, (2) setting, (3) narrative that cover my gripe with the series.
1: Characters
The characters are perhaps my biggest gripe and the biggest flaw this manga has. Aside from a few that tickle my fancy (Saito, Felid, Crowley, and Krul; actually most of the vampires), the rest fall into standard cookie cutter molds (i.e. character tropes). Now before I go any further, character tropes are not in and of themselves bad but there is a reason I said "cookie cutter molds" and that's because OnS's protagonists are just that: molds that have nothing to mold. It's as if Kagami Takaya took a character type for a character.
Guren and Yuuichiro are perhaps the most blatant examples of this. Guren's characterization is played out as some dark and twisted hero but the truth of the matter is he comes off as a villainous idiot. And Yuuichiro? Yuuichiro has as much depth to him as a toilet bowl. He's 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.
Guren on the other hand is a hard case. It's hard to root for Guren not because his actions are what led to the start of the series but because his characterization makes it so hard to root for him. There is little remorse or guilt for his actions shown early on and we only see a tinge of this maybe 30 chapters in so it sort of dehumanizes him even if the reason for his actions are entirely human. Finding out he's been manipulated since the beginning and he's fighting back against that does very little to make him admirable. Hell Berserk's Guts is much easier to root for even if he has killed children because even though he commits such obviously heinous things, his remorse reminds us that he is human (though that remorse can not in and of itself justify his actions). Guren on the other hand is a hard-ass pragmatist with a sourness on top. He's rough, ambitious, and full of cockiness with no charisma. Setting aside his fantastic character design (hell the majority of the characters have great design), his lack of charisma is perhaps his second greatest flaw. For a leader character like him, I fail to see him like a leader and more like a puppet (albeit true in the sense that Mahiru more or less directs his actions). So he talks big but acts and looks like a dick.
Mikaela is as bad. He has little character other than trying to protect Yuuichiro and drag him away (though current chapters indicate he might be hesitant in continuing this). (Refer to Part 3 of this review in case of the argument that his character is explored in the spin-off.) The rest of the gang also suffer from a lack of characterization (lol Mitsuba) as stated by another reviewer (again refer to Part 3 of this review). 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.
2: Setting
A lot of people seem to enjoy the worldbuilding and I can't exactly deny them this. The setting is interesting and the beautiful art helps to convey it (something about dilapidated and destroyed buildings, ruined oceans, and monsters running amok make me remember some of Godspeed You! Black Emperor's albums) but the thing is it's cluttered when we begin. We have (1) vampires, (2) a virus that threatens extinction, (3) angels, (4) monsters, (5)demons, (6) God, and (7) magic all from the get go but the biggest issue I have is between (1), (3), (4) 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, it's good to clearly demarcate 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.*
3: Narrative
This is less of an issue than the first but still a major one. Kagami Takaya clearly set up a complicated narrative for OnS 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 OnS but in OnS'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 OnS on the grounds that it's characters, especially its protagonist and deuteragonist, are so poorly characterized, and its narrative so splintered. If you enjoy typical chuuni narratives with a shallow protagonist and poorly developed characters but with an intricate plot, OnS is a readable (albeit tedious) choice.
*EDIT: While I might have been dismissive in my earlier comments, I am still open minded to seeing what Kagami Takaya has in store for us.