After it was announced that Willem Dafoe was cast in Justice League, there was much speculation of who the actor could be playing, from Darkseid to even an aged up Hal Jordan. However, thanks to the set visit of Justice League, we now know who he is playing. Dafoe is playing an Atlantean named Vulko.
Vulko in the comics is an advisor to Aquaman and typically portrayed as a good friend to the King. He helped support Aquaman throughout his life and towards his goal of becoming King. Vulko is the chief scientific advisor in the comics, but is also heavily involved in the Atlantean politics.
In The New 52, Vulko is a key character in returning Arthur Curry to his rightful place on the throne. After he couldn’t prove that Ocean Master killed his mother, he was going to be killed until he escaped to the mainland and lived in Norway, until Arthur Curry found him.
Vulko appeared several times throughout Aquaman titles, so it’s safe to assume that while his role in Justice League will be minor, his role in Aquaman will not be.
Justice League will be released in 2017 and stars Ben Affleck, Gal Gadot, and Henry Cavill.