Kaya Scodelario as Supergirl/ Kara Zor- El
Imagine a Supergirl TV show which carries on DC's tradition of being dark and broody. Now imagine Kaya as an alien constantly on the run, always moving from one place to another so that the Department of Extra-Normal Operations doesn't capture her. Unlike Kal- El, she didn't arrive on Earth as a baby, she arrived as a 17 year old so she knew that the Earth isn't ready to know that they aren't alone in the universe. She also knows that the DEO knows of her existence so she tries to maintain a low profile. She constantly questions her existence and wonders if she should be patient or show the world what she's capable of.
Dylan O'Brien as James Olsen
A young, struggling reporter who is also a major geek. He loves sci fi and conspiracy theories. He is intrigued by the supernatural because of which he constantly pursues leads to wild and odd reports and tips in hopes to find evidence that aliens exist. The fact that he chases such stories is why people don't take him seriously and he wonders if he should drop them, until he meets Kara.
Sean Bean as Hank Henshaw/ Cyborg Superman
A CIA agent who works for DEO. He is a strong, no nonsense character who does not trust aliens. He follows one rule: if he can't explain it, he doesn't trust it. He is hellbent on bringing Kara down, not because he's sadistic but because he knows that someone which her capabilities cannot be allowed to roam free. He does not let his emotions get in the way of his job. Further, his determination leads him to volunteer for Project Cyborg, which grants soldiers super strength and several other enhanced abilities.
Jessica Alba as Alex Danvers
Another CIA agent with a soft side for Kara, who works alongside Henshaw. Once Kara gets captured by the DEO while protecting Jimmy, Alex helps her escape and lets her lay low in her house for the time being.