Supergirl definitely has no shortage of strong, independent women in its storylines. Kara and Alex are two of those amazing characters, and although they're strong in their own ways, they need each other to truly reach their full potential. Although they aren't bonded by blood, they're bonded by their love for each other, and the way they support one another in any situation. Let's take a look at one of the best sister duos we've had the pleasure of knowing.
When we first meet the Danvers sisters, Kara has yet to assume her role as Supergirl. She's starting to think about using her powers, and her sister is not loving that idea. Alex sees herself as Kara's protector, and as her older sister, is worried about shielding her from the consequences of revealing her true self to the world.
This identity that Alex holds for herself is a theme that follows them both, and an idea that Alex has to grapple with later on. She has to learn that she's more than just someone to protect Kara, and that can't define her entire life.
Ironically, the trigger that causes Kara to leap into action using her powers is when Alex's life is in danger. Alex's plane experiences technical difficulty, and it becomes the first plane that Kara saves using her Kryptonian abilities. Although her life was saved, Alex isn't impressed by the display of affection. She's worried that Kara put herself in danger trying to save her, and as previously stated, the protecting is supposed to be Alex's job, not Kara's.
Kara doesn't understand why Alex is so upset, and it causes a rift between the two. Miscommunication is hard.
Yet another case of miscommunication (or lies, or hiding everything from your sibling) comes when it's revealed that Alex works for the DEO, a fact that Kara was obviously not aware of. This comes as a big betrayal, as Alex has kept such a large piece of her life a complete secret from her sister.
Kara eventually learns to forgive her, but it definitely isn't instantaneous. Discovering that your sister hunts down aliens like yourself has to be a hard thing to come to terms with.
After a while, Alex and Kara are on good terms again, and Alex becomes more supportive of Kara's role as Supergirl. Alex loves Kara so much that she takes it upon herself to be her fighting coach. Imagine that for a second, being the fighting coach for Supergirl. Pretty intense. The walls are laced with kryptonite so that Kara is stripped of her usual super-strength, but still.
Alex clearly cares about Kara's wellbeing, and wants to support her in her newfound duties as Supergirl.
When Kara is attacked by a parasite, Alex is there to save her. Kara is trapped in a dream world that appears to be Krypton, and the parasite has lured her into thinking that it's reality. It's Alex's willingness to go into a dangerous situation that really shows her loyalty to her sister.