Bruce Banner knew all along that he had a daughter, but it was unbeknownst to the other Avengers because he ended up putting his child up for adoption because he couldn't take care of her and he didn't want her growing up around a mutant. Little did he know, 16 years later, his child would be making headlines as a mutant herself and he would be forced to take her in and bring her up in a more suitable environment for mutants.