Miss Doris Kilman is in love with Elizabeth, an attraction that Clarissa finds repulsive. So repulsive, in fact, that Clarissa thinks of Miss Kilman as a monster – eek. Clarissa is of course afraid that Miss Kilman will take Elizabeth away from her; that's how moms work. More importantly, though, her repulsion comes from the fact that, surprise surprise, Miss Kilman sits at the opposite end of the social scale as Clarissa. She’s poor, single, overeducated (the horror!), and ugly. She has worked for everything she has, including the frumpy green mackintosh that serves as a reminder of her poverty.
Having lost her job for not agreeing that all Germans were villains, Miss Kilman lacks the blind patriotism that would allow her to fit in. As you can imagine, that makes her pretty darn lonely. Her life is a struggle in every way, which leads her to resent all of the privileges that Clarissa takes for granted. The social gulf between Clarissa and Miss Kilman is huge, and it doesn't look like that's changing anytime soon.