Beulah is Robert's late wife. She recently died of cancer, and her family lives in Connecticut. She met Robert the same way the narrator's wife did, by answering an ad for work, and accepting a job working for him. We don't know much else about her, but she is important to the story. For one thing, her recent death puts the reader in a position to see Robert as a sympathetic character, especially when the narrator begins using her as fodder for some of his worst jokes, assumptions and opinions. Here's an example:
She told me a little about the blind man's wife. Her name was Beulah. Beulah! That's a name for a colored woman.
"Was his wife a Negro?" I asked.
"Are you crazy? My wife said. "Have you flipped or something?" (1.11, 12, 13)
Beulah reveals things about the other characters. In this case the narrator becomes less likable. Interestingly, Beulah also acts as a foil to the narrator. Like the narrator, Beulah had to deal with the relationship between Robert and the narrator's wife. We have no idea how she felt about this relationship. Was she jealous like the narrator, or did she support the friendship?