Friendship could be a theme. Maybe the story is about how someone formed a strong bond with the girl upstairs, through shared interests or difficult times.
Another theme might be self - discovery. The girl upstairs could be on a journey of finding herself, and those around her are influenced by her growth and changes. For instance, she could be trying new hobbies, exploring different career paths, and those in her vicinity are witnesses to her transformation, which in turn makes them reflect on their own lives.
One possible theme could be isolation. If the girl is locked upstairs, she is cut off from the outside world, which can be a very lonely and terrifying experience.
One possible theme could be fear. If the man upstairs behaves in a strange or menacing way, the babysitter might be afraid, and fear would be a central theme.
One possible theme could be isolation. Being locked upstairs implies being cut off from the outside world. Another could be power and control, if someone locked her there against her will.
One possible motive could be jealousy. If there were other people in the household who were jealous of the girl for some reason, like her beauty or a special inheritance she was set to receive, they might lock her upstairs to keep her out of the way.
I'm not sure specifically which 'Girl Locked Upstairs' true story you're referring to. There could be various real - life or fictional accounts with such a title. It might be about a girl being held captive upstairs in a building, perhaps in a case of abuse or illegal confinement. But without more context, it's hard to tell the exact details.
One possible theme could be survival. The girl in the basement would have to fight to stay alive, whether it's against the elements, lack of food, or an abductor.
One possible theme could be isolation. Being in the closet implies being cut off from the outside world.
One possible theme could be self - discovery. The girl might go through experiences on the mountain that make her learn more about herself.
One possible theme could be survival. If she was in the bunker, she must have been trying to survive whatever situation led her there.
One possible theme could be survival. If a girl was trapped in a basement, her struggle to stay alive through difficult conditions would be a significant part of the story.