Captain Arthur Hastings is the best friend of Hercule Poirot.
He was once a military captain during World War I, but was wounded and returned to England to become Poirot's assistant and an amateur sleuth. He also befriended Miss Lemon, Poirot's secretary.
He is extremely brave and often used by Poirot for physical duties such as catching and subduing a criminal. Poirot likes to tease Hastings about being dim-witted at times, but he clearly enjoys the Captain's company.

Hastings represents the traditional English gentleman -- not too bright but absolutely fastidious, a throwback to the Victorian era gentleman who is always concerned about "fair play". Unlike Poirot, who is not above lying, surreptitiously reading other people's letters, eavesdropping, etc., in his quest to solve a case, Hastings is absolutely horrified by such things and usually refuses to do these things even when asked to do so by Poirot.

(Source: Agatha Christie Mistery Wiki)