Sirens, according to Greek mythology, were the daughters of Aqueloo and a Muse. They are creature half woman and half bird. They had a very melodic and smoothing voice that no sailor could resist them, they could challenge the Muses in a singing contest.
Here, the Muses emerged victorious, and because of that sirens started have their precious bird wings. Sirens, after such a humiliation decided to flee to the coast of Sicily, where it is said that even reside. To avenge the affront of the Muses, they attracting sailors circulating in that area, they sing a beautiful songs. The sailors who hear this song could not avoid hearing, and approach them to die while the ship crashing against the rocks near the island. In our culture when we speak of a siren normally imagined with a creature where the top of his body is a woman and bottom is a fish like mermaid.
Siren appear as a very useful GF in Final Fantasy VII, with it's technique "Silent Voice" will give the enemy bad status for a long time of period.