In some instances, Gengetsu (along with her sister Mugetsu) have been shown with Nue Houjuu from Undefined Fantastic Object. Early into the Lotus Land Story Extra Stage, there is a ball of light that, like any other enemy, tries to stop the heroine from continuing. Since Nue was the ball of light that attacked the heroine in stages 4 and 6 of UFO, that's probably the reason why.
Fans depicted her with an insane and bloody personality. This maybe because the name of her Music Theme "Cute Devil~ Innocence".
Many fans compare her with Flandre Scarlet for this fact, and in some Fanworks are seen together.
Or sometimes with her sister Mugetsu, both covered in blood.
Since her sprite sheet had shown one of her legs invisible in the game, some fans had also copied the style that her legs are shown incomplete.