Seiren was a Cardian released by Ail and An with the mission of gathering energy from humans in order to revive the Makaiju. Her main target was Rei.

She resembled a pink haired mermaid, and could use her long hair to incapacitate her enemies and drain them of their energy; she absorbed the energy into her hair, which made it turn blue, or into her golden scales.

She attacked by firing a stream of water at her target. This attack managed to neutralize Sailor Mars' Fire Soul, but proved useless against Fire Soul Bird.

(Source: Project WikiMoon)