Arrow: 8 Best Villains
6. Black Siren
Despite what a certain obsessive section of the fanbase may say, easily one of the biggest mistakes Arrow has ever made was killing off Laurel Lance in the show's fourth season. It wasn't just a disservice to Black Canary's comic legacy and Laurel's immense growth throughout the show, but a disservice to actress Katie Cassidy, who consistently gave the show her best even when the material shortchanged her.
Fortunately, The Flash did its best to mitigate the loss of Laurel and Cassidy by quickly introducing Black Siren, Laurel's evil doppelganger from Earth-2, near the end of its second season, who has been a thorn in Team Arrow's side ever since. Unlike Laurel, Laurel-2 is a metahuman, providing a pretty powerful physical threat, and her mere existence has proven to be a psychological threat as well, as several people, particularly Quentin Lance, have been unable to shake the fact that she isn't "their" Laurel.
Though she's currently a part of Cayden James' evildoing team, she recently set Quentin free - with tears in her eyes - after being ordered to kill her doppelganger's father, and it's hard to imagine her future won't hold some sort of larger redemption. At the very least, it's good to have Cassidy back in the fold, relishing every moment she's on screen, as her inner battle between staying evil and doing the right thing has proved to be one of the highlights of an otherwise rocky sixth season.