While I have absolutely no interest in meeting in real life a professional assassin that runs a protection racket, drug dealers, prostitutes and fences as a mid-level crime boss, I don’t mind coming across one in books. There’s an old favorite series of mine by Steven Brust that has just such a character. Vlad Taltos is an unrepentant criminal. What the character has going for him is a witty observation of the world around him that reminds me a lot of my favorite series and character Harry Dresden in the Dresden Files.
Vlad Taltos is a human, or as the local inhabitants of his home city prefer, an "Easterner". He lives in a vast fantasy city peopled by Dragaerans who are all taller, stronger, much longer lived and more magically inclined than humans. The Dragaerans divide themselves into 17 houses, each with their own set of talents and traits. Mixing bloodlines between the clannish houses is nearly taboo. The only house that will take mixed house members in is the Jhereg. The Jhereg will sell anything to anyone without scruple including a minor title to a social-climbing Easterner. Vlad finds he has a talent for beating up Dragaerans and decides it suits him much better than working in his father's little restaurant paying protection to the nearest Jhereg thug. Better to claw his way up to neighborhood boss himself! The first three books by Steven Brust about Vlad can be found in The Book of Jhereg. As all the titles are based on made up animal names and the series is very long, I recommend checking Novelist Plus for series order.
If swords suit you better than a scoundrel, I also have loved the Tiger and Del books by Jennifer Roberson for years. This series being from the 1980s it includes a common trope in fantasy at the time of having the heroine be the one and only woman warrior in a men's world. In these books, a lot of these characters felt not very female, but Roberson’s novels are an exception. The characters also age and change as the series matures with time. I like my novels character driven and Tiger and Del are interesting, well-developed characters throughout. The first two books can be found in The Novels of Tiger and Del Volume 1. Again, check Novelist Plus to get the books in the right order.
Some final words in favor of these books: I have room on my personal shelves for no more than 2000 books and I usually have hundreds less than that. I've held onto these complete series since 1983 and 1986 because they're good enough to rate keeping for decades. Even though you can see the decades on the first book in their stylistic choices (and I've gone from seeing them with a child's eyes to an adult's perspective), the interesting characters and the authors growing and changing their writing style as the decades pass by make these both fantasy classics in my books.