Who is Captain Arthur Hastings from Curtain and what is their importance?

Captain Arthur Hastings is the novel's narrator and one of Poirot's best friends. He is a widow with three grown children. Hastings previously lived in Styles, and while he lived there, a murder took place. Now that three more people are dead, Hastings wonders if the house carries a curse. According to Poirot, Hastings is very one-sighted. He takes what he hears at face value and doesn't dig or investigate what he hears or sees to determine that various possibilities. He tends to focus on one and stick to it.