For the Justice League animated series, the well-known (but under-appreciated) superhero Aquaman was left out of the initial lineup and was replaced by Hawkgirl, the team's second female (along with Wonder Woman).
Shayera Hol, known on Earth as Hawkgirl, was a founding member of the Justice League, and a member of the Thanagarian military. She played a key role in the Thanagarian invasion on both sides.
During the Imperium invasion, Hawkgirl was telepathically summoned by J'onn J'onzz to assist in repelling the invaders. Hawkgirl was teamed with Superman to take out the aliens' factories, which propelled smoke into the air to block out the sun.
Due to her efforts during the invasion, Hawkgirl was invited aboard the Watchtower, and formed the Justice League along with Batman, Superman, Wonder Woman, The Flash, Green Lantern and J'onn.
Like all modern Thanagarians, Shayera Hol had a pair of wings growing from her back which allowed her to fly. As a Thanagarian, her physical strength, endurance and durability were also noticeably much greater than those of humans.