Alex Danvers

Alexandra "Alex" Danvers is an employee of a secret government organization and the foster sister of Kara Danvers.

Arrival of Kara
When Alex was 14, Kara Zor-El was fostered by her family. Though Alex was jealous of Kara's special abilities, she was curiously fascinated, leading her to learn as much as she could regarding aliens.

Adulthood
Around twelve years later, Alex had begun working for a secret government organization.

Trivia

 * Alex Danvers is the only main character so far, who doesn't originate from the comics, as she was created exclusively for the TV series.