Sirene was once the lover of Amon until he had combined with Akira Fudo and was forced to become Devilman. Outraged and concerned that other humans might successfully merge with demons, she came to Earth in order to slay Devilman and serve him up as an example to any other humans that dared to try melding with demons. Accompanied by the demons Ghelmer and Agwel, Sirene launched an attack against the Makimura residence in order to force Akira into a confrontation. Akira killed Agwel and Ghelmer, but when he lowered his guard, Sirene struck. Her talons prevented Akira from unleashing his powers as they sank into his flesh and she took flight, smashing Akira against several obstacles and gloating over how she will torment him in Hell. As she gloats, she is suddenly struck by a rifle round fired by Ryo Asuka and is forced to drop Akira, who is then able to transform into Devilman and fight back.
Devilman and Sirene do battle in the air until their fight takes them into the forest. Using her projectile claws, Sirene causes Devilman serious injury, but Devilman turns the battle around when he uses his antennae to grasp Sirene's, taking control of her flying talon and impaling her with it. Realising that she's losing the fight, Sirene calls out to Satan and pleads for reinforcements. Satan seems to answer her pleas as several demons appear in the forest to attack Devilman, including Kaim. Kaim urges Sirene to merge with him, but Sirene is hesitant to do so because of her grievous injuries. Kaim, however, doesn't give her a choice and after confessing that he has loved her since the dawn of time, rips his own head off. Sirene then melds with Kaim in order to preserve him, but her consciousness remains dominant after they meld. While her strength is briefly renewed by the merger, her wounds are shared with Kaim and result in both their deaths. Before dying, Sirene made one last attack against Devilman and appeared to have killed him. As she exulted in her triumph, Sirene succumbed to her wounds and her body became frozen in place, and so she died believing she had won.
Sirene is capable of flying at great speeds and possesses a certain degree of psychic ability. She can tap the minds of others using her antennae, can project telekinetic blasts and can even fire own limbs as projectiles, controlling their movements telekinetically. Her talons contain some kind of neutralizing magic or toxin that saps her prey of their strength; in Devilman's case, it prevented him from transforming and using his powers. As with all demons, Sirene can meld with others of her kind, as she did with Kaim just before her death.