Kaji is a young boy who appears in a story where he seeks the assistance of Ginko, a traveling mushishi. His primary concern is his mother, Saigo, who is suffering from a progressive and uncanny memory loss. The cause of her condition is a mushi known as kagedama, which resides within the brain and feeds on memories. This affliction has a profound impact on the family's life, as Saigo gradually forgets more and more of her past and her daily experiences, though she consistently holds onto the memories of her son, their home, and how to cook.

Kaji is defined by his deep bond with his mother and his quiet endurance in the face of a situation that has no simple solution. He is a dutiful and caring son, taking on the responsibility of managing their household while witnessing his mother's slow decline. His motivation is straightforward but heartfelt: he wants to understand what is happening to her and, if possible, find a way to stop it. When Ginko explains that the mushi is incurable and that the only course of action is to continuously create new memories, Kaji must come to terms with this difficult reality.

In the story, Kaji’s role is that of a witness to an irreversible change, embodying the experience of a loved one left behind. He does not possess any special abilities to see or interact with mushi, making him a normal human at the mercy of their inexplicable influence. His key relationship is with his mother, and the narrative places its emotional weight on the strength and endurance of their connection. Even as Saigo loses most of her memories, Kaji finds a measure of solace in the fact that she has never forgotten him. This realization becomes a cornerstone of his development, guiding him from a state of anxious searching to a position of accepting a new, difficult normalcy. He learns to value the constancy of their bond over the painful losses caused by the mushi, embodying a quiet and pragmatic resolve.