Rio Conwy
The narrator of the novel. She is a sixteen-year-old girl living in Atlantia; she is Bay's twin sister and Oceana's daughter. She is also a siren, a person who can use her voice to make others do her bidding. She is tough and somewhat secretive, and longs for adventure in the Above.
Bay Conwy
A sixteen-year-old girl living in Atlantia; she is Rio's twin sister and Oceana's daughter. Bay is known for her generous and kind personality. She is extremely close to her sister, but shocks her by making the decision to go to the Above.
Minister Nevio
The Minister after Oceana. He is much more cunning and dishonest than Oceana, and forces Rio from her position at the temple by showing her notes from her mother's journal. He is secretly a siren, and more interested with maintaining his own position than protecting Atlantia.
Justus
A priest in the temple, and a friend of Rio, Bay, and Oceana. He is kind and gentle. He heard Rio use her siren voice at the ceremony, and began avoiding her afterwards.
Oceana
Mother of Rio and Bay. She was the Minister of Atlantia until she was found dead at the doorstep of her sister Maire. She was compassionate and generous, and some people even believed that she was capable of performing miracles. She made Rio hide her siren voice.
Fen Cardiff
A young man who chose to travel to the Above during the ceremony. He is later revealed to be dating Rio's sister Bay, and to be struggling with a severe case of water-lung.
Josiah
Head machinist of the mining bays and Rio's boss.
Bien
A young woman who works as a machinist. She is efficient and caustic. She takes special joy in tormenting Rio about her mother's death.
Elinor
A woman who works as a machinist. She is described as quiet and kind, and she has great respect for Oceana and the traditions of the temple, even believing that she became a deity after her death.
Hali
A friend of Rio who works at the temple. She attempts to shield Rio from gossips after her sister goes to the Above.
True Beck
Best friend of Fen Cardiff. He first approaches Rio to tell her that he saw Fen and Bay together, and thinks they left for the Above together. He makes mechanical fish. He is exceptionally kind and gentle, and eventually he and Rio fall in love. He has the ability to tell when sirens are lying.
Aldo
Owner of the swimming lanes in the deepmarket. He is focused on making money.
Cara
Owner of a small stall in the deepmarket. She bought Oceana's ring from Fen Cardiff and charges people money to obtain its blessings.
Maire
Aunt of Rio and Bay, and sister of Oceana. She is one of the most powerful sirens in Atlantia, and enjoys special privileges from the Council. She has a number of powers that are unique among sirens.
Ciro
The Minister of the Above. He is sympathetic to the people of Atlantia, despite the fact that most of his people would be happy to see them die.