Most people in the realm believe that Tommen is Cersei and Robert’s son, though his actually Cersei and Jaime’s biological son. He is also Joffrey and Myrcella’s brother and Twyin’s grandson. Ned’s discovery that Joffrey, Myrcella, and Tommen are not Robert’s children sets in motion Ned’s plan to ensure that the Crown passes to Stannis Baratheon instead of Joffrey.