Sirens were apparently created during the initial Chimeran invasion of the United Kingdom in 1950. The strain were first reported in the destruction of British Central Command; in which Sirens entered the flooded chambers of the base and made quick work of the human survivors, tearing apart and eating the terrified occupants. According to a surviving soldier's account, he notes that some survivors went willing into the waters, as if under some sort of trance.
By the events of Operation Overstrike, Maquis mercenary James Grayson began encountering Sirens during his rescue of Raine Bouchard in the Bonn conversion facility. Despite sharing similarities to the Fury that were encountered by the Americans, Grayson believed that the Sirens were a newer or at least separate breed of aquatic Chimera.
- Because Sirens can only be found underwater and in water-like substances, it is important to know that only energy weapons will work against them. Only the Auger WS, Longbow 1S-1K, the BM001 and BM003 Razors are capable of functioning underwater and dealing damage to them.
- The Advanced Auger-FS is also extremely useful against Sirens, as its shield is indestructible and will reflect projectiles back at the enemy who fires them.
- The Siren is named after the dangerous bird-women who sang enchanted songs to sailors which lured them to shipwreck on rocky coasts.
- Sirens were originally known as "Hydrids" or "Giant Fish".