The Sirens are half-human half-bird creatures of Greek mythology. They await among the cliffs and rocks near to sea, and sing with their beautiful voices. Sailors who heard their singing will be enchanted, driven to leap into the sea and drown or steer their ships into the rocks, shipwrecking them. Accounts of Sirens are mentioned in the tales of Jason and the Argonauts, as well as the Odyssey.
"Origin: Mediterranean. Half-human, half-bird fairies of Greek myth. They live on isolated islands and charm passing sailors into wrecking their ships. They were turned to stone by the famed lyre player Orpheus on the Argo expedition."
Siren appears as a non-playable character in the surface world; she is a relatively harmless demon and is not fought. The Messians seized her from her home in the Abyss and imprisoned her in a tower in the surface world so the workers at the Factory would remain hypnotized and senseless to the inhuman conditions they were toiling under, mindlessly working while paying no heed to their surroundings.
After defeating Belphegor, it is necessary to go to the Abyss through the "backdoor", a man named Petersen is there, looking for Siren. He was a bard who unexpectedly aided her to escape from the band of ruffians using her song to lure ships to crash into the rocky cliffs near their base. Once escorted to the Siren, they will meet up and return to the Abyss.
Siren appears in the 16th chapter of the manga. She used her magic to make the heroes see anyone but themselves as Anatomy in order to make them kill each other. The illusion is broken when Reiji Kido, who isn't affected due to the attributes of his Persona, intervenes by wounding Siren and eventually killing her using Bres' Twin Slash attack.