Supergirl, also known as Kara Zor-El, is the female counterpart to Superman. Also a Kryptonian, Kara is actually the cousin of Superman (although, in some continuities, Kara is the adopted cousin of Superman rather than blood-relations). She grew up on the Kryptonian colony, Argo, but was sent to Earth/was rescued by Superman after disaster struck Argo. Superman took her in and proceeded to train her in combat and her new powers (although, in some continuities, Kara is also trained by Wonder Woman). Ironically, Supergirl is technically older than Superman, being a teenager when he was born. However, she was placed in stasis when Argo was devestated/on her trip to Earth, awakening decades later to a now older Kal-El.
Being the only other living Kryptonian in the universe as well as the younger cousin of Superman, Kara must put up with her cousin's overprotectiveness. She possesses all of Superman's abilities, including flight, super strength, heat vision, superhuman stamina, super speed and reflexes, x-ray vision, microscopic vision, telescopic vision, and invulnerability. However, Kara lacks Superman's experience and level of fighting prowess. She is also vulnerable to Kryptonite and magic, like Superman. Nevertheless, she is a force to be reckoned with in the DC Universe.
