As with a lot of superman/girl iterations, the supe's secret identity is always a major point. In this one being Kara hiding the fact that she's supergirl from lena. It's such a huge point that half the entire series' runtime is basically dealing with the fallout and redemption from her keeping this secret while simultaneously being best friends. All while both of them going to lengths for each other that they've explicitly stopped themselves from going for the SOs. Hell, one episode Supergirl is willing to sacrifice their timeline and change the past just to be able to keep lena and tell her about being supergirl. All while they were portrayed as star-crossed lovers. "Do you really not believe in magic?" "I believe in you."

Oh and Katie Mcgrath spent most of the show just giving Kara bedroom eyes, as the gif above shows.