Arno Stark is the true son of Howard and Maria Stark. Experimented on by 451 during Maria's pregnancy, he was intended to be the future pilot for the Godkiller. However, Howard interfered with 451's experiments and Arno was born incapable of walking or speaking on his own. He was placed in protective custody and hidden from the world.
When Tony returned to Earth following his adventures in outer space with 451, he discovered Arno's location and the two brothers agreed to work together to make the Earth a better place.