In Fist of the North Star, Rei's hair changes from blue to white after he experiences a significant emotional and physical ordeal. This transformation is often interpreted as a symbolic representation of his character's development and the hardships he endures. The blue hair is associated with his original persona, while the white hair signifies clarity, a new understanding, or transformation following his experiences.
This change is not just a physical alteration; it reflects his growth as a character and his struggles throughout the story. In the world of Fist of the North Star, character designs often convey deeper meanings, and hair color changes are sometimes used to illustrate a character's journey or emotional state.