Sumireko is a high class, polite girl. She's considerably warmer to people than Tokaku is, but she's hinted to be one of the more dangerous members of Class Black. 

The story is set at Myoujou Academy, a private girls' boarding school. The 10th year's Kurogumi class has 12 assassins with their sights on one target, Haru Ichinose. Tokaku Azuma is a recent transfer student who is also targeting Ichinose, but she gradually develops feelings for Ichinose.
 
(Source: Anime News Network)

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Bryn Apprill is an American voice actress affiliated with Funimation

At Myojo Private School, an elite all-girls boarding academy, the Class Black is comprised of thirteen students, twelve of whom are assassins. Their prey is the thirteenth student, Haru Ichinose, an upbeat girl who vows to survive school and graduate unharmed! Enter Tokaku Azuma, a cold-blooded killer who transfers from another school to kill Haru. 
 
What should have been a routine hit turns into a far more complicated affair as Tokaku is unable to pull the trigger on Haru. As Tokaku copes with her budding feelings, will she decide to protect Haru's life from the other assassins or to end it before they do? 
 
(Source: Seven Seas Entertainment)

Unaired episode 13 of Akuma no Riddle released with the seventh Blu-ray/DVD volume.

A series of supplementary Akuma no Riddle side-stories and art, published by Sunao Minakata's doujin circle PURE, featuring multiple artists.
 Note : chapter & volume count includes vol.0

Sumireko Hanabusa