The Survey Corps finally makes it to the basement and lay hold of Eren’s father’s books. In those books, Grisha Yeager tells them the reality of life outside the walls and that the people within the walls are a race that has been damned by the world
The Ant king in his mother’s belly received nutrition from different humans, but Gyro is said to be the human it took after. Why Gyro? Maltreated as a child, Gyro willfully sought out to bring chaos & spread evil as retribution for his abuse.