Himeyuka And Rozione's Story

In a corner of the city, Himeyuka, a female high school student, begins living by herself in an average, unremarkable apartment. One day, she returns from school to see her building completely covered in colourful, crazy childish scribbles!
The offender turns out to be a little boy named Rozione who is, for some reason, incredibly attached to Himeyuka, even though she can't remember ever laying eyes on him before . . . what is his story and what does he have to do with Himeyuka?