Japp, while being a serviceable detective, is no match for Poirot; he frequently finds himself a step behind the great detective but has developed a grudging respect for the little man's abilities over their years together. Japp and Poirot's friend Arthur Hastings often commiserate on their confusion and inability to keep up with Poirot on cases.
Japp perhaps represents a more mainstream English citizen, decidedly simpler and uneducated in his tastes and material desires (as opposed to Poirot's high-class gourmet palate).