Born on Krypton, 20-year-old Kara Zor-El was sent on a rocket programmed to follow her cousin, baby Kal-El's rocket following the destruction of the planet Krypton. Tasked with looking after Kal-El on Earth, her ship had a malfunction and she was trapped in stasis for decades; finally arriving on Earth years later she awoke to discover Kal-El fully grown and acting as Earth's champion, Superman.
Their intended roles reversed, Kal-El teaches her how to use her powers responsibly while Wonder Woman takes her under training and Batman provides her with the necessities to begin a new life on Earth.
Supergirl, located in the Meta Wing of the JLA Watchtower, will initiate the Level 30 mission to locate and assist her cousin, Superman, who is currently inside the Daily Planet building that is being shrunk and bottled by Brainiac.
Supergirl is called upon by the Justice League to investigate reports of metahuman experimentation in a LexCorp-funded wing of Metropolis General Hospital. She finds extensive evidence of metahuman experimentation conducted by Dr. Psycho and attempts to liberate the meta-humans. She is thwarted and her DNA sampled by one of the new metahumans supported by the Secret Society of Super-Villains working for Lex Luthor.
Supergirl is an iconic opponent for villains in Alternate Metropolis in the Brothers in Arms operation.
While in the comics her skirt is blue, in DC Universe Online Supergirl's skirt is red; possibly taking inspiration from the Supergirl (1984) film, which in turn could have been inspired from the original "Supergirl", Super-Girl's outfit from Superman #123 (1958).