Josefina begins as an imaginary companion conjured by Santiago, a young boy wrestling with insecurity about his future. She takes the form of a blue-green whale swimming within a water glass perched on his nightstand. Though born from Santiago's vivid imagination and unseen by others at first, physical manifestations like causing an earthquake when threatened hint at a presence beyond pure fantasy. She can alter her size at will, shrinking to fit within the glass or expanding to mountain-like proportions, and possesses the power of flight.

Her core role is offering Santiago emotional support and guidance through childhood trials. She helps him navigate complex family dynamics, including his annoyance with his younger sister Rosa and interactions with his strict father, and process significant events like his grandmother's death. Acting as his coping mechanism, she accompanies him on dreamlike adventures within his mind, confronting fears and uncertainties. She reassures him during his anxiety about First Communion, even leading to a brief incident where he believes he swallowed her.

While Santiago remains her primary connection, others sometimes sense her presence. His sister Rosa sporadically demonstrates the ability to see her, a trait growing in importance later. Josefina also factors into Santiago's relationship with his first love, Celia; he shares tales of Josefina and their adventures, slowly eroding Celia's initial skepticism.

Josefina's existence and purpose evolve directly with Santiago's emotional growth. As he matures, gains confidence, and forms deeper real-world bonds—especially with Celia—his need for her diminishes. This journey concludes with Santiago no longer requiring her constant support, leading to her bittersweet departure symbolizing his coming of age. The narrative suggests her story may continue through a potential new connection with Rosa, who might now become her companion.