While Nora seemed doomed to die in Season 3 when Mallus took over her body, she was saved when Damien took her place.
Damien Darhk died (again), leaving Nora to be locked up for her crimes… or so she thought.
Nora is free and has good reason to owe Ray a favor.
Nora technically made her Arrow-verse debut on Arrow back in Season 4 as a little girl, but she reached full villain status as an adult on Legends of Tomorrow in Season 3 when she and her father teamed up to try and unleash the time demon known as Mallus on the world.