Neither the first episode nor the second episode was told. Her mother is classified as a dangerous person, so she has to be raised in the dark since she was a child, that is, she has not seen the light. Maybe her birth or her existence is a threat or even a danger to some people. However, when she reaches school age, she must receive education, so she must go to college, but she can't let outsiders know her existence, so she can't leave. As for how she studies in the library, I personally think that locking in the library means letting her study by herself, and locking in the top floor will not be found. Personal understanding ~ please adopt ~