The siren is a deadly creature, half human, half bird or fish. Early sources say the siren is human (always female) from the head to the navel, and bird from the waist down. Later sources say that the siren is fish from the waist down, like a mermaid. They usually have wings. In some cases sirens are described as having both bird's feet and a fish tail, as for example by Philippe de Thaon.

Sirens charm men with their beautiful singing. Sailors who are attracted to the singing fall asleep; the sirens then attack the men and tear their flesh. They sing when it is stormy but weep when the weather is fair.

Thomas of Cantimpré adds an unusual twist to the story: When sailors see a siren they throw an empty bottle to her, and while she is playing with the bottle they make their escape.

The story of the sirens shows that those who take delight in worldly pleasures will become the devil's prey.

Sirens are depicted as half woman, half bird or fish. Some illustrations show both bird wings and fish tail.There are siren illustrations that show a woman with two fish tails. A few illustrations include bearded male sirens. Sirens are often shown playing a musical instrument, usually a harp or lyre or pipes. The siren and the onocentaur are sometimes illustrated together; both are symbols of lust.

Sirens are sometimes shown holding a fish. In Christian symbolism the fish could represent the human soul, so the siren holding a fish may suggest that the siren does not just eat sailor's bodies but also their souls.