Odder is an adventurous and deeply curious sea otter living in Elkhorn Slough. Known to the humans as Otter #156, she possesses exceptional agility in the water and earns the title "the queen of play" among her peers. Her fearless nature often leads her to approach human kayakers and venture into riskier waters, despite explicit warnings from her mother and friends.

Kairi is a practical and cautious sea otter who prefers the safety of Elkhorn Slough over the unpredictable open ocean. As Odder's closest companion, she often tries to temper her friend's reckless curiosity. She strictly adheres to survival rules and worries constantly about the consequences of interacting with humans or venturing too far from the kelp beds.

The Aquarists are a dedicated group of human scientists and caretakers at the Monterey Bay Aquarium, a facility the otters refer to as "Highwater." They operate a specialized conservation program focused on treating, rehabilitating, and rearing injured or orphaned sea otters. To prevent wild otters from bonding with them, they deliberately assign numbers rather than names to animals they plan to release back into the ocean.

Ondine is Odder's mother, a diligent wild sea otter who spends her days grooming her pup, hunting for sustenance, and instilling necessary survival lessons. She observes her pup's restless, inquisitive spirit and names her "Odder" as a result. She carries the difficult responsibility of teaching her naturally playful offspring to fear ocean predators and humans.

The Adolescent Shark is a roaming predator in the bay driven by sharp hunger. While sharks do not typically prefer sea otters for food, his hunger and speed make him a severe threat to any slower-moving animals that cross his path near the entrance to Elkhorn Slough.

Gracie is a permanent resident sea otter living at Highwater. Because she cannot survive in the wild, the humans have given her a name rather than a number. She acts as a source of information for newly arrived otters, explaining how the human facility operates and what different human noises and behaviors mean.

Holly is another permanent sea otter resident at the Highwater facility. Like Gracie, she holds a human-given name, marking her as an animal that will not return to the ocean. She shares her tank with other otters undergoing rehabilitation or living out their lives in captivity.

Otter #209 is a highly vulnerable, orphaned sea otter pup brought to the Monterey Bay Aquarium. Lacking the skills to swim, dive, or groom herself, she relies entirely on the facility's specialized surrogacy program to survive her early weeks. Her constant cries reflect her total dependence on others for care.