Sophia is a young Finnish girl spending the summer in a cottage on an island in the Gulf of Finland. Having recently lost her mother, she experiences unpredictable bursts of grief and frustration. She is highly active and intensely curious about the natural environment surrounding her family's summer home.

Grandmother is an 85-year-old artist and matriarch who spends her summers on the island with her son and granddaughter. She uses a walking stick and takes Lupatro for severe dizziness. She maintains a fierce independence, frequently sneaking away to explore the island and create driftwood sculptures in a dead patch of trees.

Father is the quiet, diligent provider for the family, spending most of his time working at a desk or tending to the island's gardens. He struggles with the recent loss of his wife and copes by staying busy with tasks like ordering non-native plants and repairing the property. He attempts to keep Grandmother and Sophia safe by setting strict rules.

Berenice is Sophia's quiet, polite friend whose real name is Herdice Evelyn (nicknamed Pipsan). Sophia renames her Berenice to sound more sophisticated. She is easily frightened by bugs, saltwater, and the tight-knit dynamic of Sophia's family, finding solace only when drawing.

Eriksson is an unpredictable, solitary fisherman who visits the family sporadically. He dislikes actual fishing but excels at scavenging items from offshore shipwrecks. He operates by his own schedule, rarely staying for a formal meal and keeping his salvaged treasures for himself.

The Övergårds are neighbors who live on a nearby island and occasionally interact with Sophia's family. They deliver mail to Father and offer practical solutions, such as swapping their domestic cat for a better mouser.

Verner is a 75-year-old friend of Grandmother who visits the island to escape his overbearing relatives. He brings a traditional bottle of sherry, despite disliking the drink, and seeks out Grandmother for a sympathetic conversation about the indignities of growing older.

Mother is Sophia's late parent and Father's deceased wife. Though she is gone before the events of the summer begin, her recent passing drastically alters the family dynamic, leaving her daughter and husband to process their grief in isolation on the island.

Moppy is a gray kitten who grows into a feral, independent cat. He rejects Sophia's heavy-handed attempts at affection, preferring to hunt birds and sleep among dirty dishes under Grandmother's bed.

Fluff is a sweet, docile white cat who loves affection and resting in the sun. Despite having the exact cuddly traits Sophia initially wanted, Fluff's laziness ultimately frustrates the young girl.

Malander is a businessman who builds a new summer house on a neighboring island. He brings a modern, protective sensibility to the area, posting warning signs and padlocking his property, which offends the traditional communal values of the older residents.

Christopher is Malander's son. He joins his father at their newly built summer home on Blustergull Rock and is quick to notice the physical evidence left behind by Grandmother and Sophia's trespassing.

Delilah is Malander's dog. She actively tracks Grandmother and Sophia through the foliage on Blustergull Rock and enthusiastically bites at Grandmother's walking stick.

Emil is a worker operating a rented bulldozer to clear a new road near the coast. When the heavy machinery gets stuck in the soft meadow ground, he waits for the rest of his crew and brushes off Sophia's questions.