Sirens are individuals who have acquired incredible, mysterious powers, and are physically distinguished by elaborate tattoos that cover half of their bodies. It is said that only six Sirens can exist at any given time, and when a Siren dies, their powers pass on to another individual. All Sirens are inexplicably drawn to the planet Pandora at some point in their lives.
There are reportedly six Sirens in existence at the time that the events in Borderlands take place. Sirens are typically female, and will develop powers whenever they inherit them from the previous Siren. However, exceptions to the "women only" rule do exist, as shown by Troy Calypso, who manifested similar but weaker powers due to being a parasitic twin to Tyreen Calypso. It is unknown if there have been any other exceptions before Troy and by extension little is known about what allows an exception to occur.
The unique tattoos are both a defining feature and a side-effect of being a Siren. They manifest when the Siren's powers first develop, appearing on one half of the body, covering them from foot to scalp. Sirens seemingly channel their powers through the arm that possesses the tattoos, which is often seen when Maya uses her power, Phaselock (but this could be because she always holds a weapon in her right hand).
Sirens thrive on Eridium, a mineral that forms the crust of Pandora, but became abundant on the surface after the events of Borderlands. Patricia Tannis suggests that, because of their relationship with Eridium, there should be a connection between Sirens, the Vaults, and the Eridians. This connection is not between Eridium and the individual Siren, as Angel mentions that Maya has no previous connection to Eridium in ECHO recordings, though she is able to eventually gain a connection through study of her powers on Athenas.
The connection between the Sirens and the Eridians is further demonstrated in Borderlands: The Pre-Sequel!. While the player is working their way through the Eleseer map towards The Vault, it is revealed that Lilith had been following the player the whole time. The Guardians made no effort to stop her, and she says it was "almost as if they wanted her to get to the Vault".
Siren powers are enhanced through the use of Eridium. This is seen at several points in Borderlands 2; where Lilith used Eridium to enhance her phasewalk ability and heal herself. This is also seen when Handsome Jack uses Eridium to increase Angel's powers. Overconsumption of the material will have negative side effects: Lilith shows signs of addiction to the substance while Angel eventually required a constant feed in order to maintain life function due to being fed enormous amounts over a long period of time, dying shortly after her supply was cut off.
The Eridian writings in Borderlands 3 provide more information on the history of the Sirens as they are written by an ancient Siren named Nyriad. The Sirens lived side by side with the Eridians, yet neither the Eridians nor the Sirens themselves know where the Sirens came from. When a Siren dies, they can choose to pass their powers on to a specific person, or to a total stranger. The final Eridian writing in Borderlands 3 ("Warning") reveals that are in fact seven Sirens in existence, overturning Handsome Jack's claim in Borderlands 2 that there were only ever six at once. Nyriad warns that this Seventh Siren must never be found.
When a Siren passes away they either elect a successor who they pass their powers to, or they fail to name such a successor and their powers are thrown into the universe, finding a new host of its own volition. Nyriad ponders on whenever or not these powers are a blessing or a curse.
While similar in many aspects, each Siren possesses a unique power that makes them different from the rest of their brethren. Some shared characteristics between all known Sirens are the unique markings that appear to cover the left side of their body, the ability to pass on their powers to an heir and the ability to manifest wings. All Sirens are also capable of using Eridium to amplify their powers to a greater extent. Sirens can also charge Vault Keys using their powers.
Phasewalk is a Siren ability to bridge the gap between dimensions, allowing the Siren to teleport both herself and others across space-time. The ability can also inflict damage upon structures caused by small explosions the Siren enters or exits phasewalk.
Phaselock is the ability to lock an object or person in place inside a blue bubble of force. The power has also been used to infuse select Children of the Vault bandits with power, creating the Anointed.
Phaseshift is technopathy, or the ability to intuitively manipulate technology and have said technology obey the phaseshifter's commands and desires.
Phasetrance is an ability to manifest transparent parts of the human form which the Siren can use to affect the world around her. Known manifestations include six arms emanating from the Siren herself, or a large fist, or series of fists, erupting from the ground, or an astral projection. These effects can cause damage in various ways, and in certain cases are also used to restrain targeted enemies.
The unnamed power of Tyreen Calypso. This ability allowed Tyreen to absorb the living energy of all living beings to increase her own power. She demonstrated the use of this ability when she stole Lilith’s Phasewalk ability and later when she absorbed The Destroyer, becoming one with it. Her brother, Troy Calypso also possessed a form of this power but was only able to absorb the power of other Sirens. This ability was previously possessed by the Siren Nyriad, who used it to complete the Great Vault by draining the Eridians' energy into herself.
- The Sirens draw their name from the Sirens of Greek mythology; according to legend, Sirens were a group of extremely dangerous human-sized vultures with human heads whose faces constantly changed. They lured sailors to their death by seducing them with their singing.
- All sirens currently known have shown the ability to generate wings when using their powers. Lilith's wings appear when using the Phoenix skill, while Maya's appear when Blight Phoenix is active. Angel's wings are shown when her true form as a Siren is revealed. Commandant Steele and Amara do not generate wings; the former is because she never was shown using her abilities while the latter can uniquely manifest up to six arms that array themselves in a wing-like formation behind her back.
- The theme of Sirens was also presented in Borderlands comics, thus events depicted in comic series aren't part of the Borderlands canon universe.
- In the Borderlands Origins #2 "Lilith" comic, upon her father's death, a young Lilith is visited by a very old Siren who tells her of her powers and that there are others in the universe like her. She promptly dies, but the comic shows the other known Sirens, and two mysterious figures in hoods.
- In the Tannis and the Vault comics, the fifth Siren Asha greeted vault hunters that came into Fyrestone on Marcus's bus until she was caught in an explosion and fell into a coma. She went by the name "Tate" and was under the care of Tannis while in her coma. She was completely brain dead, and it is unclear whether or not Tannis had anything to do with her revival. She later awoke and helped the Vault Hunters and Tannis retrieve that last piece of the vault key from a Rakk hive. She did this by using her powers, which according to her, grant her the ability to "control the beasts of any planet".