A knight clad in dragon-like armor, with spear in hand. Though not one to display emotion, he is compassionate and possesses a strong sense of justice.—Description
Kain is Baron’s chief commander of the Dragoons. He’s also Cecil’s childhood friend, who accompanies him to the Mist Village. However, they weren’t always close; Kain thinks of Cecil as his rival, due to the perceived special treatment the latter received. However, Rosa mediates their disagreements, and the three of them grow up as friends.
Rosa’s considerate nature allures Kain, and he falls madly in love with her. Unfortunately for him, Rosa loves Cecil. His jealousy becomes the conduit for Golbez’ mind control:
Kain: Forgive me, Rosa. Not all of what I did was because of Golbez’s spell. I just wanted to keep you…to keep you by my side.
He forces Kain to steal the Crystals from Cecil’s party at random intervals while on the Blue Planet. Only Tellah’s Meteor is able to break Golbez control over his mind.
Near the end of Final Fantasy IV, he ventures to Mt. Ordeals to train alone:
Kain: Cecil…Rosa…I have no right to bear witness to your joy – not like this. I will temper myself here, on Mount Ordeals. And then, when I’ve become an even finer dragoon than my father – perhaps then I can return. Until that day comes, I can but wish you well.
When does Cosmo summon him to World B? None of Kain’s actions suggest brainwashing. He willingly betrays his teammates to do what he believes is right:
Tifa: “There is another cycle, right?”
Kain: “That I do not know myself. It’s not as though I saw proof.”
Lightning: “Yet you went around knocking allies out?”
Kain: “There was hope in my actions. Not hope–but a gamble, perhaps. I took a risk to believe in a chance, rather than live as a pawn to disappear.”
Tifa: “You should have talked to us.”
Kain: “If I did, no one would be asleep and everyone would be here.”
Lightning: “I guess…you’re right about that.”
Kain: “I could only knock them out alone… Protect them from our enemies– We are only able to attack the gate thanks to all of you.”
Even Golbez does not control his mind or actions:
Kain: “Heh. Surely you do not mean to stop me… not now, with the end so near.”
Golbez: [grunts] “I believe…I owe you my thanks. I had begun to abandon hope. I had thought the cycle endless, and Cecil, beyond saving.”
Kain: “Our work is not yet finished. If we fail, it was all for nothing.”
Golbez: “Should you go on, your undoing is assured. You are certain you do not wish to see the next battle?”
Kain: “Not all roads have a future waiting at their end. But this is mine.”
Golbez: “Then far be it from me to stop you from walking it to the end.”
Kain: “There will be an end to this. A true end. I promise you that.”
The likely answer is something during his training on Mt. Ordeals.