A Siren is an aquatic being capable of seeing broken hearts and passing their curse onto drowning humans. All sirens were once humans, now cursed to roam the seas in perpetual loneliness.
All sirens suffer from the same issue - a fear of perpetual loneliness. As such, sirens search for humans with broken hearts, sing out to them, and lure them into the water. If the human wishes to start anew, sirens can kiss the human and exchange roles. If the deal is a success, the human loses all memory of his or her past life and awakens alone not knowing who they are or where they came from.
According to Ian, competition for broken hearts can be dangerous. Consequently, sirens do not normally trust others, let alone appreciate one another's company. The loneliness all sirens feel is never-ending, however, so most of them are driven to turn back into a human as they consider this to be the cure to their everlasting fear. Nonetheless, in cases like Crim's, some sirens prefer to stay as they are since they believe nothing can be gained from becoming a human once again.
The siren's song is what sirens use to lure the heartbroken into the depths of the ocean. Once heard by the heartbroken individual, it is nearly impossible to resist the temptation to follow the tune.The Song (Episode 0):
"O broken hearted one, your soul has grown weary...
Have you turned to the rippling tides to engulf your lonely tears?
May the waves embrace you,
Lull you softly to sleep...
Ease away all the hurt and pain
That you've carried through the years
But if given the chance,
Would you forget the past?
You and I can start all over
We can love again, my dear."
- Curse: Sirens can pass their curse onto submerged, heartbroken humans if the drowning individual desires a major change in their life. This turns the siren into a human and vice versa - the ultimate goal of most sirens.
- Halted Aging: A siren ceases to age for as long as they remain a siren. Once they return to being human, they will begin to age again after one year. Should they revert to being a siren, the aging clock will reset back to one year.
- Hearts: Sirens can see broken hearts; glowing blue inside a person's chest. The brighter a broken heart glows, the more distraught the human, and ultimately the easier it is for a siren to lure them in with the curse. The heart's luminosity and duration depends on the human's emotional and mental state.
- The heart may also glow yellow or green, though this is extremely uncommon. A yellow heart means that the person is experiencing feelings of love or happiness, whereas the green heart is thought to represent something akin to jealousy (since it is the combination of both blue and yellow hearts).
- Singing: Sirens lure humans in with their singing. As shown by Ian and Tua, Sirens appear to all know and share the same song.
- Echolocation: In order see through murky waters, Sirens use echolocation from singing to enhance their eyesight. This allows them to see the ocean in vivid detail. For Crim, echolocation compensates for his blindness, allowing him to see the world around him albeit through inverted colors.
- Prognostication: Crim the soothsayer claims that he can see the future as the servant of the Poseidon.
- Erase/Retain Memories: The Poseidon is currently the only one of wiping and restoring the memories of fellow Sirens. It is done by using an inscription along her forearm.
- Change Appearance: According to the Poseidon, Lyra, which she claimed to be her sister, can change her overall appearance except for her aura.
Sirens lose all memories of their past lives upon receiving the curse, and cannot regain them without the usage of external magic. Additionally, sirens have no means of eating or reproducing.
Should a pregnant woman become a siren, they will continue to gestate. If they do not return to human form in time, the process of childbirth will kill them, as they lack the necessary organs to deliver a child. The child will be born a human, and will remain healthy provided that they are found in time.