Frank is the agent responsible for managing all affairs of Manderley, and is absolutely committed to Maxim and his new wife. Other characters present him as being boring and dull, but the narrator does not find him so. In fact, she seems to identify with Frank a lot:
He seemed glad to see me. I smiled back at him. It was nice of him to be glad to see me. I liked Frank Crawley. I did not find him dull or uninteresting as Beatrice had done. Perhaps it was bcause I was dull myself. We were both dull. We neither of us had a word to say for ourselves. Like to like.
He is a profoundly good character who obviously sympathises with the narrator in her bewildering position of being the new mistress of Manderley and having to cope with so much. Note how Frank reassures her by saying that she is precisely the kind of wife that Maxim needs. He is the first friend that the narrator forms at Manderley, and the strength of this friendship is shown when the narrator feels able to ask him about Rebecca and her beauty. His loyalty to Maxim is shown at the end of the story when it becomes clear that he knew that Maxim had killed Rebecca but chose not to say anything.