In Agatha Christie’s first published novel which of the following characters appeared?

Agatha Christie's first published novel was “The Mysterious Affair at Styles”, written in 1916, but not published until after the First World War (1920 in the USA, 1921 in the UK).

"The Mysterious Affair at Styles” is a remarkably carefully crafted book that bears re-reading a few times. It is historically meticulous, revealing a keen awareness of the changing times in which the author was writing. The plot is clever and reveals a detailed knowledge of the law. There is great attention to detail including maps of the house (Styles), the murder scene, and the drawing of a fragment of a will.

Christie's first novel is also remarkable in another respect. It introduces three characters that appear regularly in her subsequent work: the Belgian detective Hercule Poirot, his friend Arthur Hastings and Inspector (later, Chief Inspector) Japp. Spoiler alert: Poirot solves the mystery!