Riddle Story of Devil

"The world is full of forgiveness."

—Haru Ichinose, Episode 1

Tokaku Azuma is a heartless teenage assassin on a mission of death. She's been sent to a prestigious boarding school to compete in an assassination game against eleven other beautiful killers. Their heinous mission: send the innocent and cheerful Haru Ichinose to an early grave. But when Tokaku finds herself strangely drawn to her intended target, she must utilize her lethal skills to keep her new friend alive -- and Haru must place her life in the ruthless, cold-blooded hands of the very woman sent to kill her.

Akuma no Riddle (悪魔のリドル, lit. Devil's Riddle), published in English as Riddle Story of Devil, is a Japanese manga series written by Yun Kōga and illustrated by Sunao Minakata, licensed in English by Seven Seas Entertainment. An anime adaptation by Diomedéa aired in Japan between April and June 2014.

Tropes used in Riddle Story of Devil include:

Academy of Adventure^[context?]
Assassin Outclassin'^[context?]
Beach Episode: The OVA Episode 13 "Who's the Winner? (Surprise Test)", which stars the whole cast.
The Clan: There are two assassin clans which have existed since ancient times: the Kuzunoha Clan of the West and the Azuma Clan of the East. Tokaku is the heiress of the Azuma Clan, and Nio is the successor to the Kuzunoha, who adopted her as a little girl. There is also Haru's unnamed clan, which was really more of a Man Behind the Man Ancient Conspiracy that was quite possibly older than both of them.
Deadly Graduation: The graduation itself was perfectly lovely. Cherry blossoms, singing, the whole shebang. It was how they got to where they were in the epilogue that made it "Deadly". As revealed at the end of the series, Class Black is actually an elaborate Rite of Passage for Haru's clan, to test the "refinement" of her primer pheromone and see if she had what it took to be a queen bee. The test did not explicitly involve killing her best friend, but she did have to survive to graduate, and that meant she couldn't just roll over and let Tokaku kill her either. This conflict of interest had different endings between the anime and manga, corresponding with the different versions of Haru starring in them:
- The Badass Damsel Ending: In the manga version of events, Haru spread her arms in acceptance and stood firm with grim determination, declaring that nothing would get in the way of her leaving Myōjō with Tokaku, not even Tokaku or God himself. Even after she was shot, Haru still managed to make a Final Speech to convince Tokaku not to commit suicide out of remorse before finally losing consciousness. And even after all of that, she did not die, surviving on pure luck rather than titanium ribs.

"I won't cry. I won't die. [...] So... Come on. Try and kill me if you can, Tokaku."

- The Damsel in Distress Ending: In the anime version of events, Haru responded to Tokaku's notice by attempting to defend herself with a knife-- and failing miserably at it. She charged Tokaku with the knife extended in front of her in full Sparkling Stream of Tears Mode and somehow managed to not only impale herself on her opponent's knife with enough force to pierce her heart, she also failed to inflict so much as a scratch on Tokaku in the process. The reason she survived? Tenacity and titanium ribs.
Death Notification: Inverted. Each assassin gets 48 hours to kill Haru once they deliver their advanced notice, and if they fail to do so within the time allotted, they are immediately expelled from Class Black. It becomes a plot point that there are not any rules pertaining to the exact nature of the notice except that it be delivered. Isuke uses that fact to her advantage by addressing her advanced notice as Banba's.
Elaborate University High: Myōjō Private Academy integrates grades K through college into one major academic campus. The Grade 10 freshman year was divided into five classes: Class Blue, Class Green, Class Red, Class White and Class Yellow. The sixth and final class, Class Black, was only opened to enrollment at irregular intervals ranging anywhere from dozens of years apart to back-to-back.
Evil Parents Want Good Kids: Well, Tokaku's mother and aunt probably wouldn't count as evil (no more than any other character in this story, at any rate), but they were fully blooded assassins, and Karami didn't want her daughter to become a murderer like them. Mako, Tokaku's aunt, tried to fulfill her older sister's Last Request and protect Tokaku from that life, but the head of the Azuma clan -- Tokaku's grandmother, Karami and Mako's mother -- caught them and killed Mako.
Fundamentally Female Cast^[context?]
Hero Secret Service: Tokaku is single-handedly serving this role for Haru against eleven other trained assassins.
Idiosyncratic Episode Naming: The episode titles are riddles texted to Tokaku by her psychology teacher from Private Academy 17, Kaiba.
In Love with the Mark: Deconstructed. The basic premise of the story is Tokaku rejecting her mission and protecting the intended target from eleven other assassins, and there is plenty of chemistry between her and Haru. However, the story also points out the sort of bad judgment someone would have to have for this trope to even get off the ground, as well as the sort of insane skills and luck they would need to survive its implementation. As Lampshaded by Kaiba:

"I chose the person with the worst judgment. Tokaku, your life isn't worth a damn to anybody!"

Knife Fight: Between Tokaku and Isuke, both in the Opening Theme and when she makes her attempt on Haru's life.
Psychological Thriller: Akuma no Riddle deconstructs what sort of personalities and circumstances would be necessary for tropes like In Love with the Mark to function and explores the psychological implications of being a Hitman with a Heart and how a teenage girl becomes a Professional Killer in the first place.
Pun-Based Title: The Japanese title is a pun on the Devil's Trill Sonata by Giuseppe Tartini.
Romantic Two-Girl Friendship^[context?]
School Play^[context?]
They Changed It, Now It Sucks: YMMV on whether Akuma no Riddle was Ruined FOREVER, but as a psychological thriller (which it was marketed as!), the anime adaption took a clear downward turn in quality, if only because of Haru's Character Derailment. In the original source material, Haru was a Badass Damsel, and as one of the two main characters, her characterization has a huge impact on the plot. In the anime, her character was taken in a distinctly more Damsel in Distress direction, which required serious application of the Idiot Ball to justify without the story coming apart.
Transfer Student Uniforms^[context?]
Twelve-Episode Anime: Plus one Beach Episode OVA starring the complete cast, bringing the total to thirteen.
Twenty Minutes Into the Future: There are touchscreens everywhere. Even in the shower! However, the same cannot be said of the setting from the source material, whose level of technology is only as advanced as "contemporary". And aside from Kaiba's riddle texts necessitating some form of IM technology, it has no real impact on the story. If you don't count the fact that Sumireko is a Cyborg, anyway.
Unexplained Recovery^[context?]
World of Action Girls^[context?]