Now with more backstabbing!
Kain and Cecil are childhood friends, both orphans taken in and raised by the King of Banon. You can see how they think of themselves as brothers. Though Kain is portrayed as silent and aloof in the game, a hard man to get to know, Cecil has no problem getting him to talk and reminisce.
Some time before the beginning of the game, Kain must have realized Cecil had feelings for Rosa, and that they reciprocated. Kain had also loved Rosa for a long time, but he wanted his friends to be happy. He didn't say anything and tried to keep out of the way. That Cecil refused to go through with the relationship must have been agonizing, and so the seeds of what are planted. Though Kain's ambitions were thwarted in love, both he and Cecil had thriving careers in the military. Kain became the captain of the Dragon Knights at a very young age, while his friend was the captain of the Red Wings. However, during Banon's "evil" phase, the military relied increasingly on the air force, because technology inevitably leads to destruction in RPGlogic. So, Cecil and his dark knights were favored over the increasingly archaic dragoons.
Despite Kain's growing jealousy of Cecil, he does not hesitate to defend his friend. When Cecil begins to question the king's orders, and is thus stripped of his rank, Kain leaps to his side. "Your majesty," he says "I beg you to reconsider. Cecil has done no wrong." So begins Final Fantasy IV.
When it becomes clear that the King of Baron's actions are costing the lives of innocents, Cecil knows it is time to renounce his country. Kain agrees, but ends cryptically.
Cecil: I refuse to continue any more slaughter in the king's name!
Kain: I knew you'd say that. But don't worry, I'm on your side.
Cecil: You are?
Kain: I owe the king so much, but I can't disgrace the dragoons.
Cecil: So you'll join me?
Kain: Yes. But we will need allies if we are to oppose the mightiest kingdom in the land. We have to do it for Rosa, as well.
Cecil: Thank you, Kain.
Kain: Save it. This isn't for your sake.
After this conversation, Kain dissappears for quite a while. When he returns, it is at the front of Golbez's predictably evil army. You can't have a best friend named Kain and have him not betray you, now can you?
While under mind control, Kain becomes Cecil's rival, rather than his friend. But he was really Cecil's rival all along, unbenownst to Cecil and perhaps Kain himself. Whatever primal urge has been released, he is driven to claim Rosa for his own and stop Cecil at any cost, in an extremely personal manner. He challenges his friend to single comment, insults his new outfit, and is determined to prove himself superior in every way.
Kain's unique in that he betrays you not once, but twice, before eventually winding up semi-firmly on the right side of the good and evil equasion. All along, Cecil, stunned, refuses to say a bad word about Kain. Indeed, he thrusts the "mind control" defense on the party more than Kain himself does. He simply won't give up on his friend's soul. Maybe some deep part of Cecil recognizes that it could have been him. Perhaps he knows it could never have been him, and that makes it all the harder. But Cecil never lets him go. The greatest part of Cecil's heroism stems from his willingness to forgive.
Not that Kain ever asked for it. He wants to suffer, to pay for the sins he is only half guilty of committing. Cecil's easy forgiveness and Kain's determination to linger on the past are the game's final stroke. Cecil has learned that random battles can't cleanse the psyche, but Kain trudges on to uncertain glory. Trying to prove himself worthy of his father, of Rosa and himself. And, of course, Cecil.