Black Kryptonite duplicate of supergirl.
She sees supergirl as her good twin and in essence, thinks of supergirl as her twin sister. In many regards that is true. Since evil Kara was born of good Kara. Made from the darkness that was inside of her. She is every bit as real as the real Kara Zor-El and it is the main reason she taken the name of Kara Zor-El II, the second, as her name and come to call herself Kara as well though later when she inserted herself into Kara's public civilian life.
She poses as her twin sister and took the name Keira/Kira Steel for herself. Later goes by Keren Dark. She pretends to be Kara Danvers long lost twin sister when she crafted her public persona. She likes Wearing dark clothing and shades to hide her identity in public.
She is a creation of Cadmus and Inadvertently, Maxwell Lord. Due to red Kryptonite turning black inside of a powerful bomb that Supergirl tried to defuse with her heat vision, instead of her heat vision defusing the bomb, it exploded and the Red K exploded into dust shards that entered Supergirl’s skin and infected her. The red K turned black from being superheated and exploded into many fragments that penetrated supergirl's body and began infecting her.
Thanks to her allies at the DEO. They managed to purge the black Kryptonite shards from her body but the shards melted into a liquid ooze substance and escape down a drainage pipe. Later that substance would grow and turn into a black Kryptonite duplicate of supergirl. Dark supergirl was born from the darkness inside of supergirl. Darkness that was suppressed and later brought to life during her time as "Red Kara".
The seeds of Dark Kara's creation began at that point and now the darkness within Kara has been made flesh. She is now everybit as real as the real supergirl. Her behavior is exactly like Kara when she was infected by red Kryptonite but on a much worse scale.
She met her twin during an attack on a secret Cadmus facility. One that she found on her own and managed to lose Lilian Luthor when Supergirl showed up. She killed several people at the base but hasn’t really killed anymore humans since she told her twin that she was “going to be good.”
While she has cause some havoc and mischief here and there, she mainly watches over Supergirl and Alex Danvers in secret. Having something of s twisted fetish for Alex Danvers and even her own twin namely. Namely she likes making both of them feel uncomfortable when she gets flirtatious with them both. Outside of her life revolving around her twin. She has been trying to find her own path in life. She sees herself as the evil twin. The Dark Supergirl who does what the good one can not. Supergirl believes that she can be saved but Darky, as Alex Danvers has come to nickname her, doesn‘t think she wants to be saved or should.
She has a belief that if Supergirl dies, she dies too. A belief she came to have when she learn she has twin telepathy with her sister. Having all of her memories and gaining new ones whenever she is around Kara for a long period of time. It is this Fear, that Keeps her from trying to kill Supergirl in fear that her life is linked to hers and that she will die too. This has often forced her to come to Supergirl‘s aid when her good half is fighting someone who has come close to killing her. Kira is not sure what to do with herself and when she is not being bad, she is often found in clubs, dancing the night away.
(tv show version of her story. I play the comic book version as well, with faceclaim of Laura Vandervort from Smallville. )