As one of DC Comics' powerhouses, Supergirl often shows off her true power without upgrades, but becoming a Red Lantern showed a whole new side of her that isn't touched on as much in recent runs. The different Lantern Corps colors represent the emotional spectrum, with red representing anger and rage. Therefore, the Red Lanterns are often considered a very violent group. It's a shock that Supergirl would be a part of this violent group, but it turns out she fits in better than fans expected.
The most famous Red Lantern is often considered Guy Gardner, whose twist on his rage led him to be a hero and showed him how to use the negative emotions he has in a good way. While he wasn't the only hero recruited by the Red Lanterns, he was the main focus and the true leader of their Corps. However, he isn't the perfect Red Lantern, as that title goes to Kara Zor-El, who became a Corps member alongside her fellow hero and turned her rage into a force for good.
The Rage-Filled Woman of Tomorrow
In a 2014 run of Red Lanterns by Charles Soule, Jim Calafiore, Alessandro Vitti and Miguel Sepulveda, Supergirl is forced to come to terms with her anger, just like Guy Gardner. As a newly formed Lantern, and a teenage Kryptonian, Kara is going through a lot of changes that she doesn't know how to confront. But the Red Ring chooses her for a reason, and all of her pent-up anger over the loss of Supergirl's people is channeled into her Lantern powers. The Ring chooses her because she doesn't know her own power and has been stewing in rage for a very long time. As Superman's younger cousin, she feels as if she's just luggage being carried around, second to Kal-El, and doesn't get to express herself. Superman even says that she chooses to sit in her anger instead of accepting what happened to her family and what's happening around her.
While Red Lanterns makes her a force of rage, Supergirl: Woman of Tomorrow by Tom King and Bilquis Evely, shows readers how anger and exhaustion over the loss of her people is her driving force. In the epic finale, Kara is faced with the choice to kill or let her captive go, and the one thing that stops her is the one person in the universe who truly understands what she's going through. Losing Krypton has made her angry, charged her with protecting those who need saving and giving a shoulder to lean on to people like her. Kara Zor-El sacrifices so much of her time and sanity to help one young girl learn that her anger can be used to help others and not to continue a cruel line of hate.
As an adult and a teenager, Supergirl learns that anger is only a tool, and how one uses it determines whether it's good or bad. The young Kara Zor-El learns that the Red Lanterns exist to prevent others from experiencing the pain that originally led them to become Red Lanterns. This is exactly what she is doing in Woman of Tomorrow, saving people from the same fate she's suffered. Supergirl goes on a long mission for the sake of saving one child from herself, accomplishing the goal many of the Red Lanterns have, and she continues to accomplish that goal with every fight, mission, and person she saves.
Kara Zor-El's reaction to losing Krypton is different from Superman's reaction; her memories of her lost home make her angry, but she's long since learned how to utilize her pain and anger to help others. Without a family like the Kents to help raise her upon immediate arrival to Earth, she learns how to be a hero by watching others. Eventually, Supergirl's goals start to align with the goals of a Red Lantern, making her the perfect candidate for their Corps.