Carlos was unhealthy from birth. As he grew, he began to display bizarre, sometimes violent behavior. However, at age 15 he was designated Phillip's heir to the thrones of Castile and Aragon.
A considerable degree of legend surrounds Carlos. Friedrich Schiller's play Don Carlos, while loosely based upon the prince's life, idealizes his character out of recognition. And his father the King was for some time rumored to have murdered him, but modern scholars do not find this claim credible.