A mermaid dreams of living on land, but much to her father's dismay because of humanity, and she is forced to make a deal with the sea witch to lose her voice.
- Serena (Amanda Seyfried) - a beautiful young mermaid, she was 5 years old, when she goes with her mother to the surface, her mother tells her that someday she'll may find a human to fall in love, until her mother was killed, 17 years later, she now wears a green seashell bra and green fins, she still thinks about the surface, she is forced to lose her voice to the sea witch, in order to fall in love with the prince, in the end,
- Hagus (Rupert Grint) - Serena's flounder best friend, in the end,
- Shelby (Jessica Biel) - Serena's older sister, who promised to watch her little sister, before Mother's death, in the end,
- Hurricane (Anthony Hopkins) - Serena's father, who forbids her to go up into the sea, in the end,
- Chamberlain (James McAvoy) - the prince and Serena's love interest, in the end,
- Tsunami (Famke Janssen) - the main antagonist and the sea witch, she steals Serena's voice if she wants to marry a prince, in the end,
- Monsoon (Tom Hiddleston) - Tsunami's son, who would wish to have power and the whole seas, in the end,