Underneath it All
Kain, like most people, has several different dimensions to his personality. He appears to be an aloof, distant person, and while for the most part this is true, there are a lot of emotions and traits that he hides by refusing to let others get close to him.
Kain is an intensly private person. He doesn't say very muuch, and when he does say something, he doesn't always say everything he's thinking. For example, when he agrees to help Cecil betray the king's orders, Cecil thanks him. Kain's response is something like "Save it. This isn't for you." He doesn't elaborate- he's probably doing it for Rosa, or his father's memory, or his own sense of honor- but we don't know any more than what we can guess.
He has his strong points. His loyalty is nigh-unwavering, but he knows when the needs of justice outweigh the needs of patriotism. He stands up for his best friend no questions asked, and is physically brave to the extreme. He's not afriad to die should his death be necessary. He cares a lot about doing the right thing, and can be quite passionate about the things he loves.
Kain's also naturally a bit of a smart-ass. Many of his lines are insults, and more often than not they're pointed in Edge's direction. The hot-headed ninja is the very antithesis of the cold, aloof dragoon, and they don't get along very well at all. Kain also throws a few zingers in the enemies direction, although his rawest expressions of dislike are reserved for Cecil.
Kain is, however, far from unfeeling, he simply suffers from the Final Fantasy hero disease whereby emotions are treated as weaknesses. Kain has a double reason to keep his heart sealed- he's in love with Rosa, who all but belongs to Cecil. However, Kain respects Cecil just as much as he loves Rosa, and he wouldn't want to make their relationship more awkward than it already is. To spare his friend pain, he hides his feelings- leaving him vulnerable to the kind of darkness that preys on the secret desires of men's hearts. For all the cheesy tentacles and demon walls, Final Fantasy IV's true final boss is the evil found within everyone. It can't quite be defeated, only subdued. But the strong moral of Kain's story is that there's no hiding inside oneself.