cahn, here's a story I learned from a Juan d'Austria biography which will please the Philip sympathizer in you though there's no way it can be used in any Don Carlos fanfiction. So, Juan gets officially acknowledged as Charles' son (and indeed renamed Juan, he was Jerenimo/Hieronymus before) following Charles' death and a plea to Philip to do just that in his last will when Juan is 12. After acknowledging Juan as his brother in public, Philip decides to have him attend the same school as Don Carlos and Alessandro Farnese (Margaret of Parma's kid), who are all roughly the same age. In particular, Carlos and Juan are supposed to keep each other company. Except Juan (and Alessandro) are really good at school stuff, and Carlos, not so much. Carlos gets irritated. One day, when he's playing tennis with Juan and Juan wins, Carlos shouts: "I'm not talking to you anymore! I'm not talking to one whose station is so far beneath mine! You're a bastard, and your mother is a whore!"

Juan shouts back: "Well, my father is way greater than yours!"

Carlos, even more incensed, reports this to Dad in a "do you know what the bastard has said?!?" fashion. Philip returns: "Don Juan is right, and you are wrong. His father and mine was a far greater man than I shall ever be."

Juan: *swears Philip life long loyalty right then and there*

BTW, Carlos did make up with Juan; when he makes a list of his friends some years later, he puts him in it. Unfortunately, the purpose is the list is a collection of "people I might persuade to rebell against Dad with me", and he's utterly wrong about Juan who reports Carlos' first approach immediately.