- “Astrid raised her eyes to his once more. They were warm and smiling, and she felt as if she was falling into their green depths like a stone into calm seas. And then Des took her face into his hands and kissed her. It was fierce and gentle all at the same time, and it took her breath away. ”
- ―Chapter 51, Dark Tide
Astrid could once sing when she was very little. After the Mahenconnor, a ondalinian festival, she was cursed with the disability to sing. As she gets older, she is ashamed of her inability. "Who wants a mermaid without magic?" her father always said. Becca helps her confess to the others, and in the end, Astrid's voice returns to her.
Astrid appears at the end of the novel, being in the battle when she cuts off one of Abaddon's hands. Once the battle is finished, Vrăja confesses them that they are direct descendants to The Six Who Ruled. After Vrăja explains who she is the descendant of, Serafina and her have a fight, over Serafina's mother's death. She gives a hint that she has a secret of some sort, but goes to bed before she can reveal anything. In the night, 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. She is so terrified by the sight she slams her tail into the mirror, smashing it, and flees the house. She goes home to realise that she is going to be berothed to Tauno, Commodora Rylka's son. Kolfinn is sick because Rylka poisoned him, and eventually died when she delivered the final dose. She finds Desiderio in prison and helps him escape she then led him to Ludo his Uncle and he helped them escape.
After swimming South for days from the Kargford, Astrid arrives in Mørk Dal, a golbin village in the Nolrth Sea. She struggles to believe that she will be able to get Orfeo's talisman as she believes the mission is "suicidal". She goes into a goblin salvager's shop to find a mirror in which she will be able to summon Orfeo and trick him into believing she wants to help him to get his talisman. She finds a mirror and attempts to enter it, but has no luck and a woman's face, "pale and disembodied" floats into view. A vitrina tells her that to enter the mirror realm, she must state her deepest desire. Astrid cannot think of her deepest desire, then realizes it is to songcast again. She enters the mirror realm and is taken by Rorrim Drol to meet Orfeo. She arrives at Shadow Manse, Orfeo's palace, to realize he knew she was coming. Orfeo heals her voice, then teaches her many songspells. Thalassa, whom he has captured, teaches Astrid more about songcasting. Astrid contacts Serafina and her friends, telling them what seems like very useful information. When she finishes the concova, Orfeo comes out and shows that Astrid was tricking her friends, and now Orfeo knows how many troops the Black Fin Resistance have. When they get to the Carceron, Serafina thinks that Astrid has betrayed them when Astrid uses Coco as leverage to get the talismans. Astrid and Coco have really created a plan to get 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. When the six merls go 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 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, later cut into a chin-length, sleek bob, ice blue eyes, and a tail with the bold 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 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 and and her hair is loose.
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 very brave and tough as she faced Abaddon without using magic. Due to her secret, she chose not to believe it. When told about it, she was sarcastic and negative, questioning Baba Vraja. This got to the point where she got called "rude" by Becca. Also, she doesn't back down from a fight. Deep inside, Astrid was afraid and admitted that she couldn't choose how to use her magic due to her not having the ability to songcast. However, she opens up to people and becomes more warmer as she realizes that the mer world is not as cold as she thought it would be- 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 her before.
Astrid lost her songcasting ability a few days after Manenhonnor, Ondalina's moon festival, because the silver drupe in the cake got lodged into her throat and blocked her voice box. This rendered her unable to create a tune on it to turn herself and her surroundings into different colours.
At the end of the book, she manages to sing and does a bloodbind with the others, 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 and she discovers she has a 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".