- “"That's the sound of your heart," Orfeo said. "So brave. So powerful." He laughed, pleased by the thunderous noise. "Blood calls to blood, child. The blood of the greatest mage that ever lived. My blood."”
- ―Chapter 7, Sea Spell
As a child, Astrid had the ability to songcast, like all mer. However, at the Månenhonnør, a Ondalinian festival, she lost the ability to sing due to an injury sustained during the celebrations. As a result, she has faced much ridicule from other mer, especially her father. Becca was the first to realize that she lacked this ability, and helps in giving her courage so that she is able to tell the other members of the Six. At the end of the series, Astrid regains her singing voice and is able to sing once more.
Astrid appears at the end of the novel, having been in the first encounter the Six have with Abbadon, in which she cuts off one of its hands. Vrăja reveals to the six merls that they are descendants The Six Who Ruled, the most powerful mages in Atlantis, the origins of the mer. After Vrăja explains that she is the descendant of Orfeo, the mage who became evil, she expresses distress and anger at being the "bad one". She and Serafina have a fight, due to the fact that both believe that the other has attacked them in some way. Astrid begins to act as if she has some secret she must keep, but leaves the room before Serafina can inquire more about it. Overnight, she leaves and journeys home, fulfilling her place in the prophecy as 'one who does not yet believe'.
Astrid does not appear in Rogue Wave, but is mentioned various times.
Astrid appears in the prologue of Dark Tide, and while journeying home from the Iele, decides to rest in an abandoned house in Atlantica . Just as she is about to sleep, she notices movement in a mirror and sees Orfeo in the mirror. He beckons her, promising to protect her, and Astrid almost takes his hand until she sees that in her reflection, Orfeo's eyes- just empty, black holes- have become her own. Frightened, she destroys the mirror and flees the house. She goes home to realise that she is going to be berothed to Tauno, Commodora Rylka's son and that her father, Kolfinn, has become ill due to the poison administered by Rylka. He dies soon after, succumbing to the poison. Astrid finds Desiderio, Serafina's brother, in prison, and escapes with him thanks to the help of his uncle, Ludo.
After swimming south for days from the Kargford, Astrid arrives in Mørk Dal, a goblin village in the North Sea. She struggles to believe that she will be able to get Orfeo's talisman as she believes the mission is "suicidal". She decides to visit a goblin salvager's shop to find a mirror in order to summon Orfeo and trick him into believing she wants to help him which will, in advance, help her take his talisman. She finds a mirror and attempts to enter it, but has no luck. A vitrina then appears and tells her that to enter the mirror realm, she must state her deepest desire. Astrid at first, cannot think of her deepest desire, then realizes it is to be able to songcast once more. She enters the mirror realm and meets Rorrim Drol, who takes her to Orfeo. She arrives at the Shadow Manse, Orfeo's palace, where Orfeo himself heals her voice and decides to teach her songspells. However, Thalassa, the former canta magus of Miromara whom Orfeo has captured, teaches Astrid more about songcasting and offers Astrid support in her mission. Astrid contacts Serafina and her friends, telling them what seems like very useful information. When she finishes the concova, Orfeo reveals himself to have listened to the whole conversation and tells Astrid that she has just helped in tricking her friends. Now Orfeo knows how many troops the Black Fin Resistance has. When she and Orfeo get to the Carceron, Astrid acts as if having betrayed Serafina and uses Coco, one of Serafina's friends, as leverage to get the talismans. However, Astrid and Coco have formulated a plan to steal the talismans, which is revealed when, after the Carceron is opened, Astrid tosses the box full of all the talismans to Coco. Coco swims away, and Orfeo and Astrid engage in a battle to the death. Astrid decapitates Orfeo's current body, then collapses, sobbing, near the corpse, as she has killed the man who gave her voice back. When the six merls swim into the Carceron to battle Abaddon, Astrid is the one to figure out how to defeat the monster, singing a powerful dirge to send the souls inside Abbadon to their resting place. She then tears off Alma's necklace, given to her by Orfeo, and sends the pearls flying so that the souls she released may have a place to rest and be taken by Horok. In the final chapters, it is shown that Des and Astrid will be Promised to each other in a year's time.
Astrid has pale skin, long, white-blonde hair, that is later cut into a chin-length, sleek bob. She also has ice blue eyes, and a tail with the black and white markings of an orca. She is tall, with a strong nose and full mouth.
When she first meets Serafina and her friends, she wears a long sealskin vest embroidered with silver thread and scabbard made from sealskin hanging from her waist. Her hair is in two ornate braids running along the sides of her head with the rest of it let loose.
In Dark Tide, she wears a sealfur parka and her hair is in two braids. In Orfeo's materialisation of her in the mirror, she wears a gown of black sea silk, a crown of polished jet, her hair is loose, and she's songcasting.
In Sea Spell, she wears a black sea silk gown which is trimmed with pieces of polished jet at the neckline and hem and has long sleeves ending in points, a nipped waist and a long, flowing skirt. After passing out, she wakes up in a sea flax nightgown.
- “She's dead. Face it. The death riders probably got her," Astrid said. "I told you this would happen, Sera. Back in the River Olt. You're asking too much of them. Will Ling's death stop you? Or are you going to keep on with the insanity until Becca's dead, too? And Ava? And Neela?"”
- ―Astrid challenging Serafina
Astrid has shown a great dislike to Serafina and even accused her of trying to assassinate her father, which isn't true. In response, Serafina also believed Astrid and her father were trying to take over the entire mer realms and believed them to be in league with Traho. However, she later discovers the truth and slowly gets over her grudge for Serafina and they become firm friends.
- “She looked at him as he broke the kiss, scared he would do it again, scared he wouldn't.”
- ―Chapter 51, Dark Tide
During the time that Astrid escaped from Ondalina and helped Desiderio escape with her, they became friends and developed an "easy, teasing way with each other." She soon falls in love with him. During the search for the black pearl, she tells Des that she can't songcast, and he is upset, but the anger is quickly replaced with his affection for her, when he kisses her.
Astrid doesn't appear to have any sort of particular dislike to Neela, although Neela believed Astrid was in league with Traho. However, in Deep Blue when Astrid cracks a joke, Neela laughs along with everyone else and in Dark Tide Neela is delighted when Astrid is back.
Astrid and Ling had an argument in Deep Blue, which resulted in Ling calling Astrid a "tingju", or jerk, but Astrid reveals in Dark Tide hat she actually liked Ling a lot and they get on when they meet up in the Karg.
Astrid doubted Ava in Deep Blue but in Dark Tide it is revealed that Ava likes Astrid and they get on and Astrid wants Ava to have hope in her mission.
Astrid was annoyed by her keen attitude to learning and her negative attitude made Becca call her 'rude'. Becca thought Astrid was a hothead and selfish because she deserted the mermaids at the Iele's caves. But Becca eventually becomes one of her closest friends, mostly because she was entrusted to Astrid's deepest secret.
Astrid didn't believe Baba Vraja about Abbadon or the Iele and left a day after. She also mocked Baba Vraja when she presented them with the mission of killing Abbadon, calling it impossible.
- “She’s blunt, difficult, and a bit of a hothead, but also courageous and loyal, and loves to ride hippokamps and orcas.”
- ―Jennifer Donnelly describing Astrid
Astrid is shown to be both courageous and reckless as she faced Abaddon without using magic. Due to her secret, she chose not to believe that she was the descendant of the most powerful mage of the Six who Ruled, Orfeo. In fact, when it was revealed, she was sarcastic and negative, questioning Baba Vraja. This got to the point where she was called "rude" by Becca. She never backs down from a fight if she can help it. However, Astrid was afraid and admitted that she couldn't choose how to use her magic due to her not having the ability to songcast. Eventually, she learns how to open up to people and becomes warmer as she realizes that the mer world is not as cold as she thought it is- like Ondalina.
She put up a tough and rude front because she thought nobody would accept her, but it is revealed Astrid is helpful and eager to learn when she opens up to Becca and eventually makes friends with Sera.
She has a very brave soul, and Desiderio says that she's "one of the strongest merls [he's] ever met." She seems very strong on the outside, but thinks of herself as very weak due to the things that have been said to and about her early on in life.
Astrid lost her songcasting ability a few days after Manenhonnor, Ondalina's moon festival, because a silver drupe in the cake traditionally eaten then got lodged into her throat and blocked off her voice box. This rendered her unable to sing at all, though she can still speak normally.
At the end of the series, she is able to sing and performs a bloodbind with the others, partially gaining Neela 's ability to do excellent illuminatas, Becca's talent at waterfire, Ling 's omnivoxa ability and Ava 's ability to sense things before they happen.
During Sea Spell, Orfeo heals her voice, then teaches Astrid several powerful songspells, thus leading her to discover that she has a rather powerful voice.
- "Astrid" is the modern form of the Ancient Scandinavian name "Ástríðr", which derives from the Old Norse elements ás "god" and fríðr "beautiful, beloved", meaning as a whole "beautiful god" or "beloved god".