Usually have loyal servant sidekicks, like Whimsey’s valet Bunter and Campion’s man Lugg. The aristocrat can move about in upper-class society where he is an equal and also can successfully maneuver in middle-class society because they are in awe of him. The faithful and keenly observant servant is necessary to investigate the lower and servant classes where the aristocrat is mistrusted.
This type has an extraordinary capacity to understand human nature and to pick up clues that others miss. They are often pitted against the less-than-brilliant police force and manage to make much of something as simple as a matchstick that would be discarded by official investigators. He is very often brought in by a person who is dissatisfied with police procedure or who wants a crime investigated without publicity.
Crime seems to find these people and they are usually just the sort of non-descript kind of people who can blend in among the bystanders and ferret out the criminal. They are often unexpectedly bright people who are always underestimated by the perpetrator.
Detectives who understand medicine and law thoroughly and who are frequently called on by Scotland Yard as expert witnesses and to perform expert analysis of evidence. Often gain a reputation as investigators in their own right and are called on to solve crimes by private citizens as well as the police.
Younger female detectives are usually representative of the “new woman” of the twenties and thirties. She is spunky and fearless and surprisingly (according to male characters) intelligent and freshly unconventional. The older female detectives , such as Miss Marple, are often nosey but kindly old-biddy types who notice everything and who can put two and two together.
This is self explanatory, really. These are official police detectives. They are sometimes on-duty at the time of the crime but are very often off-duty but on-hand when the crime is committed. Sometimes they are out of their jurisdiction but are called in to help by the local constabulary.