Isana is a young girl who appears in a seaside village where Ginko, the wandering Mushishi, investigates rumors of rebirth. She is the daughter of a woman named Mio and lives with her mother and grandfather. Isana has black eyes and black, neck-length hair, with a tanned complexion and typically wears a yukata.

Her existence is the result of a peculiar and intimate relationship with a Mushi that resides in a deep oceanic chasm known as the Ryuuguu. Isana was not born under ordinary circumstances; she is the reborn version of Mio's own mother, Mana. Terminally ill and afraid of ceasing to exist, Mana willingly went to the Ryuuguu to be reborn. Following this, Mio agreed to consume one of the red eggs produced by the Mushi, which caused her to become pregnant with the exact physical replica of her mother. While Isana is biologically and physically identical to Mana, she is a distinct individual raised by her daughter, Mio, from infancy.

Isana's personality is shaped by this unusual origin. She is perceptive and appears to be a normal, if somewhat quiet, child. However, she is keenly aware of the tension surrounding her identity. She is bothered when others, particularly her grandfather, call her by her former name, Mana. For Isana, being recognized as herself and as her mother's daughter is of paramount importance, and she finds happiness in the fact that Mio calls her Isana. As she grows, her mannerisms and appearance increasingly mirror those of Mana, which creates a deep emotional conflict for Mio, who struggles to see her own daughter rather than her own mother.

The central role Isana plays in the story is to embody the complex and ambiguous nature of the rebirth phenomenon. She forces both Ginko and the viewer to confront philosophical questions about identity, memory, and what constitutes a person. When Ginko investigates the Mushi, he concludes that Isana is physically identical to her grandmother but, having been raised by Mio, is fundamentally her own person. This distinction becomes critical when Mio is swept out to sea and nearly consumed by the Mushi that created Isana. In a moment of recklessness and love, Isana dives in to save her mother. When Ginko later reprimands her, noting she could have simply let Mio be reborn and then given birth to her again, Isana rejects that idea. She states that she would prefer her mother to die as the person she knew and loved, with all her experiences intact, rather than be reborn as an empty copy. This choice represents a departure from Mana's original fear of death and affirms Isana's own philosophy that a life's memories and relationships are more precious than mere physical continuation.

Isana does not possess supernatural abilities in the conventional sense. Her significance comes from her origin as a product of a Mushi that can restore an organism to its fundamental embryological form. The red eggs from the Ryuuguu contain embryos of all animals, and the Mushi effectively consumes the days of a being's life before recreating its physical form. Isana is the living result of this process. Her ability, if any, is the unique perspective she holds—as both daughter and mother to the same person—which allows her to make a definitive choice about the value of identity and the natural cycle of life and death. Her development culminates in this firm resolution to protect the person her mother has become, rather than the genetic blueprint she represents. Her key relationship is with Mio, defined by a mutual love that is complicated by grief and the blurring of familial roles. In the epilogue of a later special, Mio and a grown-up Isana are seen together at the shore, suggesting they have found a way to coexist peacefully.