Seiren (セイレーン, seirēn?) is a 2nd generation, Underwater Warfare Angeloid type η (Eta) (水中戦闘用エンジェロイドタイプη（イータ), uichūsentō-yō enjeroido taipu (Īta)?) built by the Master of Synapse.
She strongly resembles the mermaids of legend with her fin. Seiren's hands also resemble fins. Her wings are floating around her.
She is abruptly killed by Chaos, who stole her powers via the Pandora System.
- The Man of Synapse is her master, as well as her creator.
- Tomoki Sakurai-enemy
- Seiren appearance is based on the Seiren of Greek mythology.
- Even though an Angeloid's weakness is water, Seiren is able to work underwater.
- Seiren can be in the deepest parts of the Ocean up to 8000 meters deep.