I didn’t actually know any of yours, astorian, but I guessed your 21.
Giving it one last shot with spoilered answers to my 19 and 20:
A former reporter and would-be pulp author who has taken to the nightlife of post-Depression Chicago.
Jack Fleming, of P. N. Elrods Vampire Files, a vampire who teams up with a private agent to solve his own murder, and goes on to become his partner in further adventures. Jack wants to write for Weird Tales.
A private detective who has put dancing and death behind her, though with less success at the latter than she’d like.
Harper Blaine, of Kat Richardson’s Greywalker series, a Seattle PI who died for two minutes, and returned to life with the ability to perceive and act in the Grey, the half-world of ghosts and other supernatural beings.
A reluctant nobleman, who perceives his family obligations to be sacrosanct. Best all around if he solves his mysteries by early evening (although he can always pick up later where he left off).His Grace, Samuel Vimes, Duke of Ankh-Morpork
A former slave who trained in Paris as a surgeon and a musician, lately returned to his childhood home in New Orleans. He’s aided by an opium-addicted violinist and an unprepossessing policeman from Kentucky.Benjamin January, Barbara Hambly’s A Free Man of Color
A former legionary who has taken up with a senator’s daughter and does undercover work for the Emperor Vespasian.Marcus Didius Falco, series by Lindsey Davis