Jess Martin is the second female protagonist of the story. She is a point-of-view character first in the chapters that feature her diary in the 1998-99 timeline and in the chapters that describe her actions during February 2019. Jess’s character arc in the novel mirrors that of the other two protagonists, Lucy and Mary, emphasizing the themes of transformation and female community.

Jess is described as having dark hair, dark eyes, and a Goth image. In the 1998 timeline, she is 16 and feels different and isolated from her peers. Her small family contributes to her sense of isolation, as does what she’s told is her skin condition, an allergy to water. Jess’s unconscious wish to be near water, which manifests in sleepwalking, reveals her deeper understanding that water is her natural element, where she belongs. At first, her choice of clothing seems a way to disguise her skin and keep herself from being noticed; Lucy notes that “Jess seemed bent on concealment, from the trick-mirror imagery of her paintings to her uniform of sweeping, shrouding black” (73). However, her skin difference is not the full explanation. Lucy notes “the distance she’s created around herself, her protective force field” (74), the result of the secret she’s been keeping all of Lucy’s life.

Jess is artistic and loves painting, for which she has a strong talent. She is initially attracted to Cameron Hennessey, her art teacher, not only because he is young and good-looking but because she believes he sees a special quality in her. To him, Jess feels beautiful. She has one close friend, Max, to whom she loses her virginity, but Jess withdraws from him when she thinks Max was disgusted by her skin. She lets Max believe that Hennessey is the father of her child because she’s afraid she will hurt Max. When Jess realizes she is pregnant, it brings a new focus to her life; she knows immediately that Lucy will be like her and could be her companion. However, the circumstances of Lucy’s birth force the family to adjust, and her parents adopt Lucy and raise her as their own child. Thereafter, Jess feels both drawn to Lucy and hurt by the lie, which accounts for the ambivalence Lucy senses.

Returning to Cliff House and painting the women who haunt her dreams is Jess’s attempt to make peace with her past and the discovery that she was Baby Hope. Though the Wilsons, now Martins, have always cared for her, Jess wants to know where she came from and why she was left, and returning to the cave illustrates The Pull of Familial Relationships. When she gives birth in the cave, she arrives at her own understanding of what the sea means to her, but she doesn’t have the freedom to truly live as she wants until she is fully reconciled with Lucy. When she realizes Cameron is guilty of the crimes he’s accused of, Jess simply wants to be rid of him; she realizes now that he took advantage of her when she was a child, highlighting her growth and maturity in the years between her two narratives. Jess achieves justice for herself, reconciliation with her daughter, and integration into her family, making hers the most complete character arc over the course of the novel.

Mary Kissane is the third female protagonist and point-of-view character. Her timeline begins in 1800, when Mary and her twin sister Eliza are accused of attacking Byrne, who works for their landlord. It made no difference to the court that Byrne had sexually assaulted Mary and that Eliza had stopped him; both girls were sentenced to transportation to New South Wales. Mary is grieved to leave their father, Da, who raised them on his own after their mother disappeared when the girls were five years old.

Mary’s character arc over the course of the novel involves recovering the self-knowledge that she has suppressed, which is connected to the loss of their mother. The story Eliza tells of the merrow woman enchanting the fisherman is the story of how their parents met and fell in love, but when their mother wanted to take the girls back to live in the sea, Mary refused. She was acting at the time on Eliza’s behalf, but she has felt guilt ever since. Mary is deeply protective of her sister, though Eliza often perceives more than Mary does, highlighting Female Connections as Protective Influence as both sisters support each other equally.

Mary begins the book as a young woman who does what she is told and submits to authority, but with the combination of Byrne’s attack, her and Eliza’s sentence, and witnessing how the captain treats the women on board ship, she begins to believe in her own understanding of what is right. At first, her body’s transformation frightens Mary, until the shipwreck jolts her into an understanding that her body can survive in the sea. Her sense of loyalty to the women who have become her friends leads her back to the Naiad to save what survivors she can. Thereafter, Mary becomes one of the titular sirens, dispensing her version of justice to men who have harmed women and pursuing motherhood in her own fashion, by having an affair with a fisherman whom she perceives as a kind man, different from the others.

Eliza plays an important supporting role, though she is not a point-of-view character. Her function is to provide motivation for Mary’s survival and to support and guide Mary toward her final transformation. As Mary’s twin sister, Eliza is a foil and complement to Mary. She arouses Mary’s protective instincts but also guides and teaches her sister, perceiving things Mary might not. Eliza is the one who understands all along that they are a merrow’s daughters, and she knows that Mary chose not to return to the sea with their mother because she knew Eliza didn’t want to leave Da. When Eliza led Mary to the stream, she was trying to prompt Mary’s self-discovery. Although Mary often thinks of her sister as needing protection, hitting Byrne on the head with a stone when he is attacking Mary shows that Eliza is equally protective of her sister.

Eliza is proof and testament to Mary’s loyalty, and during the shipwreck, when Mary returns to assist the humans, Eliza aids her. Eliza’s face is the image that haunts Jess, likely because of Eliza’s importance to Mary. While there is no mention of Eliza in the Epilogue, which reveals the nature of Robert and Mary’s relationship, the reports around Comber Bay have been of multiple women singing. This suggests that Mary and Eliza have enjoyed their freedom, their sisterhood, and their shared mission for quite a while.