Billy Mar-Vell is actually a young boy who, when he shouts the word “Kree!”, transforms in the super-scientifically enhanced adult hero named Captain Marvel. His fellow members in the Judgment League Avengers are not aware of this secret.
After Angelhawk, Canary, and Goliath found Super Soldier, Captain Marvel soon joined and formed the Judgment League Avengers. He was transported by Dr. Strangefate with other members of Judgment League Avengers to battle Thanoseid. When Mariner was accused of blowing up oil freighters the U.S. government branded him an eco terrorist and Captain Marvel and other non metamutant members of the JLA captured him and imprisoned him without a trial.  When the other meta mutant members of JLA found out about Mariner they broke him out under Mister X but Captain Marvel and the other non metamutant members of JLA were not going to give up without a fight.