this is an old comment, sorry about that.
you could say that about rock in "marine express", where he was a hero and a good guy. you have to remember that the tezuka star system is like an actor taking on roles in different movies; rock was like a child star that grew up and took on darker villanous roles (though even as a child, he seemed to get the darker more serious roles even if he was the good guy/hero).
i haven't read or watched princess knight yet but i'm not sure where you're getting the "they look nothing alike" from! at least, in the anime shots ive seen, rock and frank look very similar! they have black hair and the poofy round bangs.
sorry to bother you.