Sirene is often described as one of the most beautiful demons, to the point where she has been mistaken for an Angel more than once. Like the harpies of Greek mythology, Sirene looks like a hybrid between a human and a bird: She has the general appearance of a beautiful young naked woman with penetrating blue eyes, light skin, slender physique and voluptuous breasts. Along with these human traits she has a feathered tail, two antennae on her forehead, bird claws as arms and legs and a large pair of white wings sprouting from her head which allow her to fly. Bird-like plumage grows around her face and over her crotch. Depending on the artwork, her plumage excluding her wings are either white or pink.

As a human, Sirene is seen as an extremely beautiful and voluptuous woman, she is slim and curvaceous, has pale radiant skin, long platinum blonde hair that falls down her back, and piercing blue eyes.