Ragnar Baratheon is the only trueborn son of Robert Baratheon, twins grew in the womb of his mother Cersei but both came from a different father. And luck was not on Ragnar's side as Joffrey was born first and therefor is the heir to the Iron Throne. But even if his father is cold towards his children and whores around he gave him a strong name Ragnar, the name of a warrior.