Although not a main character of The March by E. L. Doctorow, Emily Thompson is certainly a memorable character mostly due to the reader's pity of her as both an assistant and lover to one of the more prominent characters in the novel.
Emily Thompson can be seen as a symbol of the aristocracy of the South. She is displaced and distant. Further, she is most prominently seen in regards to her relationship with Colonel Sartorius. Emily Thompson is both the assistant to Colonel Sartorius as well as the lover of Colonel Sartorius. Her love-making with the Colonel is passionless, allowing the reader to be full of pity for this minor character.
To augment her character, we can look closer at her lover: Colonel Sartorius. He is a field surgeon in the Civil War and, as such, has become numb to its horrors. In fact, Colonel Sartorius simply carries his saw everywhere, ready to hack off limbs at the slightest provocation. It takes a certain woman to be a "passionless" lover of this kind of man.
In conclusion, we can peg Emily Thompson as both a minor character and an interesting character in Doctorow's novel. A prominent symbol of the displaced aristocracy of the South, Emily stands as a sad testament to the Confederacy.