I dunno, I think the underlying crux of his powerlessness as being the younger brother who craves the ability to rule an army while constantly getting shut down by his father provides a really great character conflict. On top of that, while he starts to revel in his ability to kill indiscriminately, he's constantly undone by his sister, who wants to keep seeing him as some pure person.
He's likable to me because he's got so many shades to his character -- at times he's romantic, obsessive, cynical, murderous, perverted, gentle, stoic, depressed. He wants what he can't have, an interesting contrast to his father, who has all that he wants but constantly craves more.
That brings me to Rodrigo, who is likable because Jeremy Irons.
