Riddle Story of Devil is based on a relatively recent manga (it only had two collected volumes in print by the time the series aired in the Spring 2014 season), but despite the dearth of source material it tells a complete story, one with a clear and largely satisfying resolution at the end and one which certainly feels like it was planned that way from the beginning rather than retrofitted because the producers were running out of material to animate. That it is intact is one of the biggest plusses for a series that otherwise stands on shaky ground.

The basic premise is certainly enticing: one teen girl must battle to protect another from a succession of female killers. Yuri romance is strongly implied, although for most of the series the episode content is much more coy about how much of the Tokaku/Haru relationship is actually romantic than the visuals in the opener are. (One other relationship in the series is clearer on this point.) Because the premise is something probably better-suited to a movie or short OVA series than a one cour TV series, the concept is padded out by having most episodes focus on the prospective killer who is taking her turn, so we get an exploration of each girl's background and her particular personality and tricks. And to the credit of both the original manga-ka and the series writer, a serious attempt is made to flesh the girls out so that they are not just crazed killers. Oh, to be sure, one or two of them actually are the classic crazed psychopaths, but the backgrounds, motives, approaches, and levels of enthusiasm demonstrated by the killer wannabes are quite diverse. (One is the adopted child of gay parents, for instance/ one of whom is an assassin unbeknownst to his spouse!) Meanwhile Tokaku tries to figure out why she agreed to protect Haru, why Haru was the one targeted in the first place, and what's up with these maddening riddles she keeps getting sent by her overseer which seem to have no clear answer – which is, of course the point: she is supposed to find her own answers.

Thus the pieces are all in place, but the execution is where the story comes up short. Action scenes are rarely as exciting as it seems like they should be, dramatic scenes often fail to be as compelling as they want to be, and the chemistry between Tokaku and Haru that is supposed to be central to the series' concept never materializes as strongly as it needs to for sustaining the concept. Although the structure is there, the flow of events feels more forced, more manufactured, than organic. An explanation does eventually come for why Haru is being targeted, and it does at least somewhat make sense within the wacky logic of a world where a class of girl assassins could actually happen, but it is still more than a little hokey. Really, though, the biggest failing of the series comes down to one thing: despite the efforts to detail her background, Tokaku just is not all that compelling as the female lead, and the whole business with her mental block is more irritating than dramatic.

That the animation is not really up to the task of supporting the action scenes also does not help. Fight choreography is nothing special, and diomedéa and director Keizou Kusakawa simply did not have the budget to make most of them look very dynamic; only towards the end does this improve. The visual aesthetics work a little better, as the layout of both public and private areas of Myojo Academy offer some interesting architectural design and some subtle but sharp stunts are pulled with color, especially hair and eye color contrasting against otherwise drab backgrounds. The girls tend to have sharp features which can turn practically demonic when some of them put on their psychotic faces, including one girl who seems capable of freely manifesting a wicked grin full of pointed teeth and one who is at one point revealed to have full-body tattoos. Haru has a definite moe appeal, while Tokaku has more of a tomboyish air. For a series about assassins, the graphic content is actually not especially strong, and fan service is kept to a minimum.

The musical score is a stronger effort throughout, with a suitable dramatic sound heavy (sometimes a little too heavy) on orchestration but not entirely dependent on it. Opener “Sosho Innocence” by Maaya Uchida (a prominent seiyuu in other titles, though she has no role here) has a nice rock edge which fits the series well, while each of the first 11 episodes has a different closer particular to the girl featured in that episode and sung by the seiyuu for the featured character. The closer for the final episode brings the whole cast together, albeit with less impressive results. Although some of the actresses seemed to be having fun playing up the nastier sides of their roles, none of the vocal work was particularly impressive, either.

In the end Riddle Story of Devil is not a bad series, and it does wrap up well, but it never comes together smoothly enough to be accurately called a good series, either. It was a concept that had a lot of potential but could not fully capitalize on it.