What’s Monka’s contribution to the development of the plot?

Monka serves as one of the story's antagonists. She is spiteful, rebellious, and we can infer that much of this has its roots in jealousy. Monka's treatment of Eulalie seems tp have as much to do with preconceived notions as it does the ways in which Eulalie is different.

As an American, Eulalie has been raised in a different culture and environment. Ato's family judges her on this basis. First, Ato married without their knowledge; second, he married a woman they consider beneath him (a descendant of slaves); third, Eulalie is completely ignorant to their ways and customs.

When Eulalie acts in a way the Monka considers inappropriate, she tattles. She verbalizes her distaste for Eulalie and is unfiltered in voicing her complaints. Note, we might also say that Monka's actions and jealousy are a direct result of her inability to sustain a lasting marriage or relationship.