Frank Crawley is the overseer of Manderley and friend of Maxim de Winter. Unlike the other characters, who the narrator often regards with suspicion or uncertainty, Frank is regularly described or portrayed with sympathy and affection. Indeed, from the moment he is introduced, it is clear that the new Mrs. de Winter feels a bond with Frank based on his role in the operation of the house and Maxim's life.
The people at Manderley and those that live in the surrounding area exist in a kind of self-contained social hierarchy. Moreover, each person's position in that hierarchy dictates their behavior and their role in the overall story. Frank, however, doesn't quite fit into the hierarchy. He participates in social gatherings and other functions with the family, but he isn't really one of them. Additionally, he is Maxim's employee; yet, his relationship with the others suggests that he is more valuable than other employees like Frith or Mrs. Danvers.
Like Frank, the narrator also doesn't quite fit into the social order of Manderley. She is Maxim's wife, but most people still think of Rebecca as his wife. Her position is such that, despite fully participating in family functions and affairs, she never truly feels like she is one of them or that she belongs.
Unlike all the other characters in Rebecca who have clearly defined roles, Frank and the narrator are both outsiders who don't quite fit. It is likely for this reason that she feels such a strong bond to Frank, as they both occupy the same awkward social space.