The Siren is an ancient creature. She uses Ivor to ensure a steady daily supply of living human hearts to sate her unnatural hunger. In return she keeps Ivor safe and the circus he is involved with in business. She is able to influence human minds and ensures all the circus attendees are happy, nearly giddy, and spend their money freely.

She colludes with Ivor in a plot to assure a permanent supply of hearts by luring Dorian to the circus and turning him over to the siren. Ivor completes his side of the deal but it backfires when Tobias won't abandon Dorian and the siren is unable to sustain herself on Dorian's heart. It is not the hearts she truly consumes but the human soul attached to it.

Ivor is defeated (likely killed) by Tobias who then saves Dorian. Together they trap the siren in an oil barrel and sink her in a lake.