Santi Costa is a young Spanish boy living in Madrid. He grows up timid, insecure, and fearful. His deep love for whales originates in stories told by his grandmother.

To cope, Santi invents Josefina, an imaginary whale companion. Josefina possesses magical abilities, including changing her size and flying. She remains invisible and inaudible to everyone else, living within a glass of water on his nightstand. Santi relies on her for emotional support and guidance through personal struggles.

He cares deeply for his younger sister, Rosa, despite occasional friction, and demonstrates protectiveness by rescuing her from an aggressive cow. His parents, Rafael and María de Costa, provide structure; Rafael enforces discipline, like after Santi resists school. His grandmother significantly shapes him through her whale tales. Her eventual death deeply impacts Santi, leading to a pivotal moment where he symbolically helps her journey to the afterlife with Josefina's aid.

Beyond his family, Santi befriends Celia in El Escorial after saving her from a cliff fall. He confides in her about Josefina, initially facing skepticism, though Celia later becomes more accepting. They separate abruptly due to his grandmother's death but reunite later in Madrid while studying for exams.

A central part of Santi's development is his decreasing reliance on Josefina as he matures. This culminates in a farewell scene where he acknowledges leaving childhood behind and no longer needing her constant presence. This progression illustrates his journey from an anxious child to one accepting life's changes, including loss and responsibility.