The following characters can be unlocked by spending the indicated number of Justice League of America Shields that can be collected throughout the game:

Aquaman: 27 Shields
Green Arrow: 76 Shields
Green Lantern (Hal Jordan): 84 Shields
Green Lantern (Kyle Rayner): 55 Shields
Hawkgirl: 70 Shields
Huntress: 53 Shields