Sebastian, who Brie later learns is actually Prince Ronan of the Seelie Court, is one of Brie’s love interests in the novel. His role shifts in the novel’s final chapters to a secondary antagonist, as Brie learns that he has manipulated her into bonding with him so that he can steal Oberon’s crown; he has hidden that Oberon was his biological father and that he has Unseelie blood. This reveal means that Sebastian and Finn are brothers.

Even before Brie realizes that Sebastian is manipulating her, their relationship is fraught. Brie knows Sebastian from her time living in the human realm, where she knew him as an apprentice to the mage near her aunt’s house. (She later learns that Sebastian took the apprenticeship specifically to get to know her and earn her trust so that he could gain the crown.) He was, at the time, glamoured as a human, a scheme that he hatched to endear himself to Brie without facing her prejudice against the fae. Sebastian uses this history between them to keep Brie close even as she experiences doubts about their relationship when she sees his cruelty against Jalek and learns more about the Seelie’s policies.

Ultimately, Brie is left uncertain how much of Sebastian’s behavior is an act and how much of his affection is legitimate. At the end of the novel, she runs away from him and their new bond, wounded by his manipulations, which lead to her temporary death and the loss of Oberon’s crown.

Finn is a possible ally, friend, and potential love interest for Brie in These Hollow Vows. Finn is the true heir to the Unseelie throne. He is Oberon’s son but had his throne stolen by Mordeus decades prior. He seeks Oberon’s crown and the Unseelie throne not out of a desire for personal glory but because he seeks to protect his people from Mordeus and Arya’s dual cruelties.

Finn keeps many secrets throughout the novel, which makes his relationship with Brie consistently fraught. At first, she is uncertain of the extent to which she can trust him, as he reveals little information even in the face of her repeated inquiries. As they spend more time together, the two strike up a tentative friendship, one that is shaped by their mutual attraction. When Brie learns that Finn knew she bore Oberon’s crown and sought to take it from her, she feels betrayed. Finn insists that he will continue to fight for the crown, no matter the cost, as it is the only thing that can save his people from suffering. The two end the novel at odds with one another, though Brie does consider this betrayal less severe than the one that Sebastian committed against her, as Sebastian was her longtime friend and the person who swore his love.

Finn is deeply loyal to his people and his duty. He is particularly close with his intimate allies, including Pretha, his sister-in-law. Finn still mourns his brother Vexius’s death, with whom he had a good relationship—a contrast to his antagonistic relationship with his half-brother, Sebastian. Though Brie lacks details about Finn’s plans, she often discerns that he seems to be troubled by his choices, which urge him to sacrifice others for the benefit of his people. The catacombs, where Finn keeps the preserved bodies of those he bonded as “tributes,” to preserve his magic and his life, symbolize his regret about the cost of keeping himself alive long enough to try to save his court from Mordeus’s tyrannical rule.

Pretha is one of Finn’s allies who strikes up a tentative friendship with Brie over the course of These Hollow Vows. Though Pretha consistently offers Brie aid in the novel, such as when she helps her get past the first round of palace dismissals at the ball, Brie recognizes that this help is more to serve Pretha’s and Finn’s interests than Brie’s own. Pretha keeps secrets from Brie, though she becomes more honest about the fact that she is withholding information as the novel progresses and the two become closer. Even so, Pretha does not share her secrets directly, instead urging Brie to talk to Finn for answers.

Brie is initially jealous of Pretha’s close relationship with Finn, something that helps her identify her own romantic interest in Finn. She later learns that Pretha’s late husband was Finn’s brother, Vexius, and that the relationship between the two is familial, not romantic. Pretha also has a daughter, Lark, who has visions of the future. Pretha worries about her daughter using her magic, as Unseelie fae are cursed to have a limited amount of magic; using their powers shortens their lives. Pretha can use her own magic freely, as she is originally Wild Fae. Brie later learns that Pretha’s brother is the Wild Fae king and that Pretha was once in love with the Wild Fae queen, whom her brother married for political reasons.

Jas is Brie’s younger sister. Though Jas does appear in the early chapters of the novel, she is absent for much of the text, as King Mordeus holds Jas prisoner in order to coerce Brie into stealing artifacts from the Seelie Court. Jas thus plays a passive role in the novel; rather than doing things herself, she is framed as the objective around which Brie orients herself. When she appears in the novel, Jas is shown to be optimistic and sweet, characteristics that Brie sees as being in contrast with her own personality. At the end of the novel, Jas retains these characteristics, though she is shaken by her ordeal in Faerie. Though Sebastian offers Jas a home in the Seelie Court (before he reveals that he plans to take the Unseelie throne), Jas opts to stay in the human realm to recover from her kidnapping, even if that choice keeps her separate from her beloved sister.