Unlike her siblings Triton and Rhode, Kymopoleia was never invited to see her parents, as her father Poseidon found her presence disruptive. As a result, she wanders his abandoned domains alone. She was reluctantly married off to the Hekatonkheire Briares as a reward for him supporting the gods in the Titanomachy against Kronos. She has stated that she hates her husband, and that his return after The Battle of the Labyrinth brings her displeasure.
On their way to Delos, the Argo II is attacked by a sudden, violent storm. Percy Jackson dives into the water, followed by Jason Grace, who protects himself with a vacuum of air via venti (Dylan). They follow a green light to an old ruined domain of Poseidon, where Kymopoleia is using her powers to try and sink the ship, and lure Percy to Polybotes, who lies waiting.
While Percy is slowly suffocated by Polybotes' poison, Jason convinces Kymopoleia to help them, by promising to build both a Roman and Greek shrine, a custom banner, her own cabin at Camp Half-Blood, and an action figure. The idea of being feared and respected is enough to motivate her to slice off Polybotes' head.
Before Jason and Percy leave, she warns them that Percy's fatal flaw may cost them the world. She also confirms that Jason's plan to defeat Gaea could succeed. She also conveys Dylan's wish to be released upon returning to the surface, which a disgusted Jason agrees to.
Overall, Kymopoleia is driven by her desire to be feared. She states that her father Poseidon is constantly restricting her powers, much to her chagrin. Her heart has grown bitter after being cast out by her family, and given over to Briares like an object. After Jason promises that she will gain recognition, she softens up a bit, and even tells Jason that she finds him funny.
As seen in The Blood of Olympus, Kymopoleia is a 20-foot-tall goddess, wearing a flowing green dress, decorated with a belt of abalone shells. Her skin is white and luminous, and her hair is made up of glowing, jellyfish tendrils. Jason states that her face is too delicate looking, besides her bright eyes. She has a cold smile that looks like she hasn't perfected the art of yet. Her laugh is like that of a dolphin.
Kymopoleia presumably possesses the standard powers of a goddess.
- Hydrokinesis: As the child of Poseidon, Kymopoleia can control and manipulate water like Percy, but to a much greater extent.
- Atmokinesis (limited): As the goddess of violent storms, she can conjure storms over the seas and oceans, such as hurricanes and typhoons.
- Kymopoleia is the only female and immortal goddess to call Percy Jackson her brother and the second sibling to call him by his full name, Perseus Jackson, along with Triton.
- Like her brother they were both introduced in the last book of their respective series Triton (Percy Jackson Series) and Kymopoleia (The Heroes of Olympus Series).
- Jason promises that she will be the first minor god he takes care of when he starts enforcing their worship.
- Interestingly, Kymopoleia is the only character in either series that is compared to a superhero from Marvel Comics (Captain America). Jason and Percy, on the other hand, are compared to superheroes from DC Comics (Superman and Aquaman respectively).