Helios is Circe's father, but he is also a personification of the sun. In Chapter 1, Circe describes his dark palace, made from shiny black obsidian:
[H]e liked the way the obsidian reflected his light, the way its slick surfaces caught fire as he passed. Of course, he did not consider how black it would be when he was gone. My father has never been able to imagine the world without himself in it.
This passage gives the reader a sense of what the sun is like as a person and a father. His vanity is all-consuming. Instead of selecting a material for his house based on how well it will hold up or how comfortable it will be for all of the people living inside, he selects the material that best reflects his own beauty back to him. Circe admits that the way the obsidian "catches fire" when he passes by is impressive. Of anyone in the universe, the beautiful sun that makes all life possible has perhaps the most reason to be vain. At the same time, his deserved self-importance makes him inconsiderate. When he is away, his obsidian palace becomes dark as night. It never crosses his mind to think about what this must be like for those he leaves behind because he "has never been able to imagine the world without himself in it." When he comes home, he can burn, manipulate, and otherwise harm his family as much as he wants because they are utterly dependent on his light. This is essentially the same relationship he has with mortals as well.
Greek mythology, the basis for Circe, is full of characters who embody elements of the natural world. Natural disasters and weather phenomena, according to mythology, can be attributed to these characters' emotional reactions. Zeus, as the god of thunder, is known for throwing lightning bolts when he is angry. Poseidon, the god of the sea, creates difficult conditions on the ocean when he wants revenge. Miller leans into these personifications in order to explore deeply-entrenched power dynamics within families and broader social systems. Helios, as one of the Titans who embodies a fundamental element of the universe, seems to have more power than any single being ought to have. He and a few others sit at the top of the "great chain of fear" because the rest of the world depends on their good will for survival. Those further down the chain will do anything to court the favor of those at the top, including harming those less powerful than they are. Circe's character arc involves figuring out how to break free of the abusive cycle over which her father reigns. In the end, she cannot entirely reject him. He is, after all, the sun: all life on earth still depends on him. However, she finds ways to forge her own path in spite of him.
In Chapter 26, after gathering flowers from her old home, Circe finally tells Telemachus about her sexual assault and the revenge she took on her attackers. She personifies the flowers in this passage, turning them into a metaphor for her own voice:
The stars were very bright, and Vesper shone like a flame overhead. “I did not tell you before because I did not want it to lie between us.”
“And now you do not mind if it does?”
From the darkness of my bag, the flowers sang their yellow note. “Now I want you to have the truth, whatever comes.”
Circe claims that the flowers "sang" from inside her bag. The flowers may be magic, but they cannot actually sing. Circe gives them figurative voices that emphasize the importance of this moment in transforming her life. The flowers' voices are imbued with color that lights up the dark bag. Similarly, by confessing her deepest secret to Telemachus, Circe uses her voice to light up the darkest part of herself.
Crucially, the flowers pierce the darkness with a "yellow note." The light of Helios is gold, as are Circe's eyes and the blood of Prometheus and Trygon. Gold is a color of the gods. By contrast, the yellow of the flowers' "note" is gold without its luster. This flat color, which nonetheless has the power to cut through the darkness of the bag, is a metaphor for Circe's mortal voice. She has always been told that it makes her a lesser, flawed being. In this scene, though, it is by using her flawed mortal voice that she heals herself. No other magic has ever been able to heal the shame that set in the night of her assault.
These are the same flowers Circe once used to transform Glaucos into a god and Scylla into a monster. Circe has always believed that the flowers turn people into the truest versions of themselves. Aeëtes, meanwhile, has suggested that they are a tool to help Circe get what she wants. She wanted Glaucos to be a god, and so he became one; she wanted Scylla to be a monster, and so she became one. The novel never clarifies exactly what the flowers do. This scene suggests that both Circe and Aeëtes are correct. Retrieving the flowers finally enables Circe to be honest with Telemachus, even about her deepest shame: in this way, they help her both to reveal her truest self and also to satisfy her lifelong desire for authentic connection.