She is portrayed by Anjali Jay.
Selena originally appeared throughout the season as the villainous Dark Kryptonian, beginning with the season's fourth episode, "The Faithful," doing so as a hallucination of Samantha Arias. As the season progressed, the Dark Kryptonian revealed Samantha's identity as Reign, a Kryptonian "Worldkiller" sent to earth to "execute justice" using excessive violence, causing Earth to burn; after the villainess activated Sam's evil alter-ego, Reign.
Selena was officially introduced by name and by her position in episode 3.20, "Dark Side of the Moon," which saw Kara and Mon-El heading to Argo City to acquire a black rock that could be used to stop Reign. It was there that Kara was surprised by seeing her mother, Alura Zor-El, who was believed to have perished when Krypton fell, only to find save haven in Argo City. Kara and Mon-El met Selena and the rest of the council, with Selena expressing reluctance to give Kara what she was looking for. It was put to a vote, and Selena, with the deciding vote, approved and allowed Kara to take the black rock.
Selena later gave Kara the black rock and attended her departure back to Earth in her ship, but afterwards, Selena's heel persona returned as she put on her black cloak and later summoned Reign. The scene officially revealed Selena as the Dark Kryptonian, and following this, the evil Selena performed her ritual to allow Reign to break out of Lena Luthor's built prison.
Selena was finally stopped by Supergirl, helped by Samantha Arias and Lena Luthor and sent in prison.