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.
