I recommend 'I'm Operating on Monsters' to you. The protagonist of this novel, Guo Yi, had a dual identity. He could switch between the NPC and the player in the game, and he had a growable [Monster Space], which meant that he could make the monsters in the game grow and heal their wounds. This kind of dual identity switching played a very important role in the game. I hope you like this fairy's recommendation. Muah ~😗
Nezuko is initially 12 years old in the manga. As the story unfolds, her age doesn't change significantly in a traditional sense, but her character development is quite remarkable.