Prince Atakan Ethermore is initially Mildred’s enemy, although he grows into her genuine love interest by the end of the novel. Atakan is the prince of the Seelie fae, becoming its king, after he murders his father, Garran, to protect Mildred. Secretly, he has half-Seelie, half-Unseelie heritage, as his mother was Unseelie Queen Kalista. This mixed heritage makes Mildred feel closer to him, as she also has Unseelie heritage and has long felt isolated from her father’s community, given her status as half-human, half-fae.

Atakan is widely believed to be “heartless” due to his shallow sexual relationships. This reputation for callousness led him to be selected as the person whose blood “anchors” the magical wards between the Seelie and Unseelie fae realms. His father, Garran, believed that Atakan’s heartlessness would prevent his heart from ever being metaphorically “stolen”: the loophole that would allow the cursed wards to be broken. Atakan falls in love with Mildred despite his intention not to, but this development serves him well, as he needs her to fracture the wards. However, he is not entirely ruthless with Mildred’s fate; when his secret plan calls for him to allow Vane to abduct her, he finds that he is too possessive and protective to permit it. This protectiveness grows as he and Mildred become closer, particularly once he has tricked Mildred into cementing their mating bond. Although Atakan cares for Mildred, he is still conniving and cruel, and he will do anything to get what he wants. He and Mildred eventually decide that his ruthlessness—a measure of which Mildred shares—makes them well-suited for one another, as they are both willing to be vicious to ensure their survival. Atakan is not quite as ambitious as his emotions suggest, for he declines the opportunity to seize control of the Unseelie court in addition to his own. Instead, he rules as a stern but fair king of Seelie, taking satisfaction in his wife and daughter rather than in the prospect of more political conquests.

King Vane Ashbone is the Unseelie King and one of Mildred’s potential love interests. He is eventually revealed as one of the novel’s key antagonists. Mildred first meets Vane when the featherbone transports her to his bedchamber in the Unseelie castle. Although Vane initially tries to claim that this magical transportation proves that the two are bound by fate, she later learns that this claim is false. Instead, her arrival in Unseelie is merely a quirk of how the featherbone interacts with magical laws.

Vane puts on a show of acting sweet and kind to Mildred, and she struggles to trust his overtures, highlighting the novel’s focus on The Contrast between Harsh Truths and Gentle Deceptions. Even before Mildred learns that Vane has been lying to her, she considers his gentleness to be a sign that he is untrustworthy. Ultimately, she learns that he had her family killed in order to frame Atakan for their murders; this crime is part of his plan to damage the alliance between Seelie and the human court, and he also seeks to demolish Mildred’s affection for Atakan. However, this plan backfires when Mildred learns of Vane’s role in the matter, for in the wake of his show of kindness, she considers his betrayal to be more vicious than anything that Atakan ever did to her. Instead of securing Mildred’s love, Vane’s betrayal cements her affection for his rival, whom he later learns is also his half-brother.

Vane’s actions also explore the novel’s thematic focus on Weighing the Costs and Benefits of Revenge. Vane professes dislike of his father, the late King Vorx, who sparked the war with Seelie when his wife, Queen Kalista, left him after years of enduring his abuse. Although Vane is sympathetic toward his mother’s plight, he nevertheless feels that his father was justified in seeking retribution against Seelie, where Kalista sought refuge. Vane’s assertion to Mildred that Vorx was owed vengeance against Seelie suggests that he also sees women as possessions, just like his father did.

This position is reinforced by Vane’s willingness to use and manipulate Mildred for his own ends. By Mildred’s reckoning, Vane lacks sufficient concern for the collateral damage that may arise from his desire to defeat Seelie at all costs. She objects to his plan to hide pytherion eggs in Cloud City, as the resulting pytherion attack will lead to countless civilian deaths. Vane, however, treats this possibility as an acceptable consequence of war. In the novel’s climax, Mildred exploits a magical vow that Vane made, using it to force him to back down from a battle against Atakan that he will otherwise lose. Vane’s ego and pride push him to sacrifice many of his soldiers, all of whom would have died without Mildred’s intervention. At the end of the novel, Vane and Mildred have no relationship, but she bears him no ill will. He retains his throne in Unseelie, but only because Atakan has chosen not to conquer the kingdom for himself. Mildred assumes that Vane is well, as her correspondents in Unseelie have not reported otherwise.

Princess Bernadette, or Bernie, is Mildred’s sister. After their parents are killed, Bernie becomes Queen of Nephrym. She is Mildred’s greatest family ally, and she protests the political marriage that Mildred is being forced into. Bernie calls Mildred “butter,” an affectionate nickname that describes the color of her hair. When Mildred goes to Ethermore and is later transported to Unseelie lands, she instinctually knows that her sister will be worried. She writes to Bernie from Cloud Castle, obscuring some of the more negative details of her relationship with Atakan so that her sister won’t worry, and she regularly pleads with Vane to be allowed to communicate with Bernie. At the end of the novel, Bernie confirms her loyalty to Mildred by allowing her sister to return to Nephrym and by defending Mildred when Atakan comes to speak with his wife. Bernie only approves of Mildred returning to Ethermore when she is convinced that her sister has chosen to do so.