Regis and Clarus are Noct's biological fathers. Gladio is his half-brother from a failed relationship prior to his marriage to the king.

++Gladio hating that Noctis is the Chosen King
++Regis breaking down to Clarus when he finds out the fate of their baby
++Noctis always sneaking into their room to sleep
++Gladio spoils the heck out of his little brother.