Soul Sharing

The heroine is an ordinary girl who has just graduated and is so poor that she can only rent a shabby apartment. The male protagonist is a young soul who died in this room decades ago. No one else can see him, only she can. He could not leave the house and could only guard it. As soon as she came home, he followed her silently, not scaring her, just watching quietly. She was scared to death at first, but then she got used to it. He helped her turn off the gas that she forgot to turn off, picked up the things she dropped under the bed, and stayed with her quietly when she cried. She told him about the outside world, showed him photos, and "left" a light for him. He couldn't touch her, he could only look at her; she couldn't touch him, but she relied on him wholeheartedly.