Nathaniel’s fiancé, Clara, and her brother Lothar act as the voice of reason and rationality throughout the story. They are rarely in close proximity with Nathaniel, but they act as a sort of super-ego, a voice of reason from the distance. Nathaniel is corresponding with them via letter throughout the story and Clara continues to try and calm his mind by telling him how absurd the whole idea of Coppelius as the Sandman is. For brief moments, Nathaniel is put at ease by his dear Clara, but this never lasts long.
Lothar acts as a sort of confidant and narrative device for Nathaniel. To him, Nathaniel relates the story of the Sandman and all of his greatest fears. Clara ends up intercepting the letter for Lothar, but is this an accident? Probably not because the letter was addressed to her. What surprises me most is how incredibly patient Clara and Lothar are with Nathaniel. That probably plays into the rationality they represent. Time after time, they welcome Nathaniel back and nuture him, despite his repeated offenses towards them.
After all, it is the brother and sister who are with Nathaniel at the very end, when he turns on Clara and attacks her. Nathaniel loses his mind and they continue to observe, as they have been doing throughout, his madness and his end. At the point where Nathaniel breaks, there is nothing more that they can do for him. He exists outside the realm of reason and they can no longer commiserate, creating the final outsider. They function as a tool for examining inter-subjectivity. There is a definite comparison between Clara and Olympia and how easily Nathaniel can move between them. Are all women the same to him? It seems they are, even if they are an automaton.