Q, Don. See the Don Q. entry.

Quade, Oliver. Quade was created by Frank Gruber and appeared in Thrilling Detective and Black Mask from 1936 to 1940. Quade was very smart and a good investigator. He was known as the "Human Encyclopedia" because he'd applied his extraordinary mind to memorizing the entire Encyclopedia Americana in his youth, and then committed to memory every odd and interesting bit of information he'd run across since then. In his own immodest but not undeserved words:

I have the greatest brain in the United States, probably the greatest in the world. I know the answers to all questions. Try me out, gentlemen. Ask any question at all--any! History, science, mathematics, general interest.

Quaile, Inspector.  Inspector Quaile was created by the Australian writer J.M. Walsh and appeared in three books, beginning with Hand of Doom (1927). He is an Aussie police inspector based in Melbourne.

Queen, Ellery. Queen was created by Frederic Dannay and Manfred B. Lee and first appeared in The Roman Hat Mystery (1929); he appeared in numerous sequels in a variety of media. Queen is the son of a famous police Inspector, and grew up to become a writer and collector of rare books. He returned from Hollywood to NYC to found his own investigator agency, using his father's connections and reputation to gain access to crime scenes and evidence that wouldn't otherwise be available to him. He is very good at being a detective, but rather absentminded.

Queen, Ellery, Jr. Like the older version, Queen Jr. was created by Frederic Dannay and Manfred B. Lee. Junior debuted in The Black Dog Mystery in 1941 and starred in the eight novel "Ellery Queen Jr." series. Essentially Junior was the juvenile version of Ellery Queen, having juvenile adventures with his friends.

Queg. Queg was created by Arthur Morrison, he of Martin Hewitt fame. Queg, a deaf mute detective, appeared in Flynn's late in 1925. Queg worked in London and solved various uninspired cases (the Hewitt stories are far superior to these) with the help of his assistant, Ruff. Although they are both clever, based on the cases they solve, they, like the stories they star in, are colorless.

Quest, Peter. Peter Quest was created by John Kobler and debuted in the October 1938 issue of Detective Mystery Magazine. Quest was a "defective detective," one of those crime-solving characters (like Seekay, the first of the breed) who had some extreme form of physical deformity. In Quest's case it was glaucoma. "He was heading inescapably for etenal darkness...nothing could save him." The attacks of blindness came at the worst possible time for Quest, leading him to have a death wish; he feared blindness above all else, and "the reckless, world-weary, embittered man whose name is already a legend among the annals of New York crime," faced with encroaching blindness, acted recklessly, solving crimes and emerging unscathed despite his approach.

Quest, Sanford. Sanford Quest was created by the evil E. Phillips Oppenheim and appeared in The Black Box, a 15-part serial which appeared in 1915. Quest is a smooth and unruffled criminologist and scientific detective who uses a wide variety of gadgets of his own invention, including an “electric mind-reader," a viewphone, a pocket bomb made of a super-powerful explosive, and a "pocket wireless telephone," to fight against crime. Unfortunately, evil Lord Professor Ashleigh has created a suit which when charged with electricity leaves the wearer invisible (except for their hands). Between that and the murderous ape-like creature that he’s raised and trained, Ashleigh begins a reign of terror which is only ended when Quest and Ashleigh duel to Ashleigh’s death.

Quicksilver, Beau. Created by Florence Pettee and appearing in Argosy-All Story Weekly from 24 February 1923 to 7 April, Beau Quicksilver, "that damned dude dick," is one of the most annoying of all pulp characters. Quicksilver is a dandy and a fop on the level of Philo Vance. Undoubtedly Pettee found him attractive--she called him things like "an enigmatical crime chaser--a mercurial mysery man" too many times for one to think other than that she was enamored of him--but the reader is repelled by him. Oh, alright. He's a short little man with two powerful fists, a brilliant mind for detecting, and a languid, affected, aggravating attitude. Fine. You go and read him. See if I care.

Quill, Peter. Quill appeared in "Peter Quill," a radio serial running from 1940 through 1941. Quill was a detective and scientific genius, and was assisted by his secretary/assistant Gail Carson and his friend on the police force Captain Roger Dorn.

Quin, Harley. Little remembered today, Quin was created by Agatha Christie and is one of her better and more mysterious characters. It is a shame that Quin is forgotten about by all but the fanatics, because he's actually a lot more interesting than someone like Hercule Poirot. Quin appeared in various stories in pulp magazines, from the mid-1920s to 1951; twelve of the stories were collected in The Mysterious Mr. Quin (1930). Little is known about Quin, and he reveals nothing about himself. He is a tall, gaunt man, dark in complexion, with a sad and almost contemptuous expression on his face. He has an affinity for shadows and darkness, somehow always managing to stand in them. His purpose in the stories he appears in is as the explicator of events; he rarely if ever takes action, but he explains matters to listeners so that mysteries are solved and the guilty, however long they had evaded justice, are punished--or, alternatively, that those plotting murders and other crimes are unsuccessful. He may or may not have anything in common with Harlequin. Quin always appears when the aging gentleman detective Mr. Satterthwaite is present, and seems to use Satterthwaite as his instrument of justice; Quin explains, and Satterthwaite takes action. (Or, rather, Quin proposes and Satterthwaite disposes.)

Quin, Matthew. Matthew Quin was created by W. Murray Graydon and first appeared in The Boys' Champion Story Paper in 1903. He went on to appear in The Penny Pictorial, Union Jack, Boys' Friend Library, The Sexton Blake Library, and The Boys' Realm, as well as appearing in a collection of his own. Although Quin did cross over with Sexton Blake on several occasions, he was actually created by Graydon as an independent character, and he was popular in his own right, rather than being a Blake spin-off, as Arthur Stukeley Pennington was. He was a "wild beast agent," hired by Karl Hamrach of London to roam the world, finding wild animals, capturing them, and bringing them back to "civilization," where they would be sold to menageries and circuses. This is obviously distasteful to modern sensibilities, but back in the early part of last century people were not so enlightened.

Quin went around the world, from the Achinese Coast to Arizona to India to "Wildest Africa," in search of game. He was, in S. Gordon Swan's words, "a man of middle age, lean and wiry of build, with bright beady eyes, a bristly moustache and a complexion that was tanned to the hue of leather." He was friends with Blake, and when Quin was feared lost in Java, and was actually in life-threatening danger there, Blake went looking for him and rescued him. Quin returned the favor by shooting and killing a tiger which was about to savage Blake.

Later on, during World War One ("Behind the Lines, or, the Clue of the Crystal Phial," Union Jack #783, 12 October 1918), Blake was engaged on a case which took him to the front in France. There he discovered Quin, now wearing a British captain's uniform. Quin described what happened to him in this way:

There was no demand for wild animals; nothing doing in my line. I realised that when I heard that Bill Kaiser was having elephant steaks and shoulders of lion from the Zoo at Berlin for his Sunday dinners, and was eating hyena chops through the week. So I cut out the jungle sport and went in for potting Germans. I raised a force of irregulars amongst my pals, the toughest riffraff I could find. Picked them up at Zanzibar and Durban, Calcutta and Madras, Singapore and Batavia, and so on. Licked them into shape, equipped them from my own purse, brought them to France, and offered them to the British commandant, who jumped at the chance, and got me a commission. Quin's own, they call my lot. Lord, man, you should see them! You'd think they were a bunch of desperadoes and gaol-birds. But fight? That isn't the word for it!

Blake was not the only famous individual with whom Quin teamed up. Quin was friends with Teddy "Kermit" Roosevelt himself, and the two had a few interesting adventures together in the depths of Africa's jungles.

Quin, Sebastian. Sebastian Quin was created by Sydney Horler and appeared in several short stories which were collected in The Screaming Skull (1930). Quin was an occult detective. I'm working on finding out more about him (and, of course, reading The Screaming Skull) and when I do I'll include the information here.
 

Introduction
A. The Abbey Girls to Dusty Ayres
B. Bagley to Scott Burton
C. Orhan Cakiroglu to Dr. Theodore Cunliffe
D-E. Dana Girls to Don Everhard
F. Ralph Fairbanks to Miss Fury
G. The Gadget Man to G-8
H-I. Dr. Hackensaw to Baron Ixell
J. Jack, Doc & Reggie to Justice Syndicate
K. Calvin Kane to Kwa of the Jungle
L. Major John T. Lacy to Langhorne Lyte
M. Professor Maboul to Mr. Mystic
N. Lee Nace to Nyoka
O. Fergus O'Breen to Ozar the Aztec
P.  Penny Packer to Judge Pursuivant
Q.  Oliver Quade to Sebastian Quin
R. Ed Race to Captain Rybnikov
S. The Safety First Club to Tom Swift
T-U. Tahara to Godfrey Usher
V. Lieutenant Valcour to Norton Vyse
W. Inspector Wade to Dr. Xavier Wycherley
X-Z. X Bar X Boys to Zorro
Links.