Arrow has finally turned Black Siren into a hero. The Laurel Lance of Earth-2, Black Siren was introduced as a villainous version of Black Canary. Since her introduction, she's alternated between being a hero and a villain, and her story is building to a climax in the final season of Arrow.
Black Siren returned in the Arrow season 8 premiere, an episode that came to a shocking end when Earth-2 was destroyed in a wave of antimatter. Grief-struck, Laurel was approached by the Monitor and offered a chance to restore her world. According to the Monitor, all she needed to do was betray Oliver Queen - which, it turned out, meant sabotaging his attempt to destroy the Monitor.
Arrow season 8, episode 5 has finally settled Black Siren's arc, confirming that she now fights on the side of the angels. The episode sees Oliver and his team head to Russia in order to retrieve the plans for a weapon they believe could kill the Monitor, and as a result Black Siren is forced to work with Anatoly, Oliver's old Bratva mentor. The two have a history of their own together, and Anatoly is initially distrustful; by the end of the episode, though, he's convinced that this leopard really has changed her spots. Confirming this, the dramatic cliffhanger ending of Arrow season 8, episode 5 sees Black Siren reveal everything to Oliver and John. Heartbreakingly, that includes her knowledge that John's wife Lyla Michaels is working with the Monitor as his Harbinger.
Arrow has settled the question of Black Siren's loyalty once and for all. The simple fact is that this must have been the ultimate temptation for Laurel; the Monitor had promised to give Black Siren everything and everyone she had lost when her entire reality was destroyed. But Laurel realized that this would have meant giving up on everything good about herself, and returning to her old ways. She decided that price was not worth paying. It's a character-defining moment, and it completes her arc at last. This may even set Black Siren up for the Canaries spinoff, which is set to launch out of Arrow's final season.
The interesting question is whether or not the Monitor was telling the truth when he said Earth-2 could be restored. If this was just a deception, it's a cruel one, and Lyla was in on it. Alternatively, if he was making an honest offer, then it suggests all the realities that have been consumed by the Anti-Monitor's wave of antimatter haven't actually been destroyed at all. Perhaps it's instead the case that they've been converted into another form, one that the Anti-Monitor has control over. If that is the case, then the heroes of Arrow and the rest of the Arrowverse may find themselves facing a Multiverse's worth of the Anti-Monitor's slaves when the Crisis finally comes.