Japp

James Harold Japp, more commonly known as Chief Inspector Japp, was a friend of Hercule Poirot's from Scotland Yard. He was very smart but tended to jump to conclusions. His main appearance was in Lord Edgeware Dies, where he teamed with Poirot to solve the mystery. Other than this, he is a more minor character, though his part is expanded in many of the films where we know more about his home life and his pestering wife.

Japp's fellow policeman named Spence evolved into Poirot's assistant in many of the later novels.

History
Japp first met Poirot in Belgium, 1904, during the Abercrombie Forgery and later that year joined forces again to hunt down a criminal known as Baron Altara. They were reunited in London, where Poirot solved Japp's cases for him and gave Japp credit in return for special favors.

Appearances
The Mysterious Affair at Styles (1920)

The Big Four (1927)

Peril at End House (1932)

Lord Edgware Dies (1933), also known as Thirteen at Dinner

Death in the Clouds (1935), also known as Death in the Air

The A.B.C. Murders (1936), also known as The Alphabet Murders

One, Two, Buckle My Shoe (1940), also known as An Overdose of Death and The Patriotic Murders. This is his last appearance in any work by Christie.