One of the most well-known tales of supervillain redemption is Harley Quinn's transformation. But if it weren't for a forgotten Flash villain—Wallace West's father, Daniel West—this shift in Harley's personality might not have happened at all. Daniel was a big part of getting the insane ex-girlfriend of the Joker to give up her evil ways.
After the events of The Flash #24 (by Francis Manapul and Brian Buccellato), where Wallace's father, the third Reverse-Flash, attempted to use his speed-force powers to travel back in time to kill his violent alcoholic father, Amanda Waller essentially coerced him into joining the Suicide Squad. After Harley ended her relationship with the Joker, she would eventually run into Daniel in the New 52 series, New Suicide Squad.
Due to their shared backgrounds of violent relationships and bad decisions, the two became slightly closer. Even though, Daniel made an effort to persuade Harley that she was more than just a cold-blooded killer and that her history did not define her.
The team was sent to infiltrate a significant League of Assassins sect operating out of Turkey in New Suicide Squad #10 (by Sean Ryan and Philippe Briones). Daniel observed Harley's harmful behavior there and questioned her about her remorse or desire to do better. She retaliated by telling him that she liked being who she was and making fun of his stupid inquiry. But Daniel found out Harley lied when the League of Assassins seized the Suicide Squad.
As a kind of discipline, Harley made friends with a group of kids who were housed with her. She enjoyed using her own sense of humor to amuse the kids. She, however, mercilessly bludgeoned several League of Assassins members in front of the kids as she tried to break out of her cage. She was forced to confront her history as a mass murderer, which grieved, and destroyed her.