Katoli is the main protagonist of the 1984 Japanese anime series. Her full name is Katoli Uckonniemi, and she is a young Finnish girl living in the rural farming community of Päijänne in southern Finland during the early 20th century, around the time of the First World War. She was born in the city of Tampere, but her father died when she was three years old. Her mother, Sarah, subsequently sent Katoli to live with her paternal grandparents, Joris and Ilda Uckonniemi, on their farm. When Katoli is six, her mother leaves for Germany to find work, promising to send for her later. Shortly after, the outbreak of World War I severs all communication between them.

Katoli is characterized by her exceptional diligence, positivity, and resilience. Noted as one of the most hardworking protagonists in the World Masterpiece Theater series, she begins working for neighboring farms at the age of nine to support her impoverished grandparents. Despite her difficult circumstances, she possesses an open and kind-hearted nature, coupled with a strong sense of responsibility and a likable, cheerful disposition. She has a stubborn streak, a trait she shares with her grandfather, yet this is balanced by a deep love for learning and reading books. Her fortune in finding work is remarkably good, which is portrayed as a contrast to protagonists from similar series.

Throughout the story, Katoli's primary motivation is the well-being of her family and the hope of being reunited with her mother. This hope sustains her as she takes on various jobs, first as a herder for the Halme family, where she becomes friends with their son, Matti, and later as a domestic worker for other families in the region. Despite her youth and the hardships of the war, she remains focused on her daily tasks and her dream of a better future. Her key relationships include her grandfather Joris, whose stubbornness mirrors her own; her grandmother Ilda, who provides gentle care; and her loyal dachshund, Abel, a gift from her mother.

Katoli shows significant personal development over the course of the series, transitioning from a child laborer to a confident young woman. Her perseverance and academic interest eventually earn her a special scholarship to attend a free school in the city of Turku. Katoli's notable abilities lie not in combat or magic, but in her practical skills and mental fortitude. Her knowledge comes from a love of self-education, and her physical resilience allows her to perform demanding farm labor from a very young age. She is also noted as being particularly lucky, consistently finding opportunities for work and personal growth even during the bleakest periods of the war.