Harley Quinn is unlike any of the other characters on this list due to the fact that her first appearance wasn't in a comic book, it was on Batman: The Animated Series. She appeared in the episode "Joker's Favor" in 1992, and wasn't placed into a comic book until the following year in The Batman Adventures #12.
Since then, she's gone on to become one of Bats' best characters, and thanks to Margot Robbie's stellar performances as Quinn on the big screen, she's also now one of the most well-known of all Batman's rogues. Her origin story was widely appreciated by fans, but it wasn't safe from a few tweaks.
Originally, her origin was told in the 1994 graphic novel The Batman Adventures: Mad Love, which showed how Dr. Harleen Quinzel, Ph.D., fell in love with the Joker while she was working in Arkham Asylum. She helped him escape and became his follower/lover.
When the New 52 reimagined the character, it tweaked one thing that a lot of fans didn't much care for. The new story showed her initiation into the Joker's embrace by having him throw her into the same vat of chemicals that turned him into the Clown Prince of Crime. (It was later revealed in Death of the Family that the Joker had also kept "other Harleys" before Quinn came into the picture.)
Some fans liked the change, and it was reflected in the film Suicide Squad, but many didn't like it at all.
Jonathan is a graphic artist, illustrator, writer, and game designer. Jonathan retired from the U.S. Army in 2017 and enjoys researching and writing about history, science, theology, and many other subjects. He writes for ScreenRant, CBR, NerdBastards, Listverse, Ranker, WhatCulture, and many other sites online. You can check out his latest on Twitter: @TalkingBull or on his blog: jonathanhkantor.com