Black Manta
Who is he? David (no one knows his last name), the archenemy of Aquaman for abandoning him to be abused as a young child. He saw Aquaman as the master of the sea and is determined to kill the former so he can control the latter himself. At one point, he killed Aquaman and Mera’s son, and is in even a father himself to one Jackson Hyde, who would later become Aqualad. (If you watched Young Justice, Aqualad is Kaldur’ahm; Kaldur was made specifically for the show, and the comic brought him in.)
Why include him? Manta has sadly been pretty underused in both current comics and outside media. But the man is a stone cold badass, both in and out of his battle suit. The original Injustice was lacking in villains that came from Aquaman’s side of the woods, so Manta would be a good start in showing folks that the King of the Sea fights more than whalers. But maybe don’t make his helmet so wide, because it just looks stupid.