It is generally assumed by the majority of fans that Koakuma is Patchouli's summoned familiar and personal assistant, as she is rarely seen outside of the library unless accompanying the magician, and served as Patchouli's midboss in EoSD. She's also frequently depicted as a succubus in fanworks, despite ZUN calling her more of an imp and her exact species being unknown.
Unused sprites in the EoSD game files include a possible "EX-Koakuma" with violet hair and a purple arcane aura, which might have been meant to appear alongside Patchouli during the Extra Stage midboss fight.
In Whispered Oracle of Hakurei Shrine, her ability has been revealed as "capable of finding target book in an instant", which can be interpreted as she listens to the books' "whispers/sounds" and triangulates their location like some kind of bat sonar. In the same work, It is also states that she is like a magical creature similar to Patchouli and the various fairies, which is in line with ZUN's definition of Devil, where he describes them as 'large species that can include vampires and magicians among other similar things'.
