To Make Amends
Kain is Cecil's best friend, and they were raised like brothers as orphans in the King's castle. While Cecil joined the more popular Dark Knights, Kain joined the Dragoons in emulation of his father. Both were quite successful, at the game's beginning, they are both captains of their respective regiments.
Kain has a sarcastic streak and justice doesn't weigh as heavily on his mind as it does Cecil's. Still, he is a good man and is ready to stick up for his friends at any cost. He loses his rank defending Cecil, and later he agrees to continue in Cecil's treason. While Cecil is sorry that Kain was demoted for his sake, Kain doesn't seem to mind. They clearly have an amicable relationship, and Kain seems willing to help drown Cecil's sorrows in drink and good times.
Of course, though Kain genuinely loves Cecil, he also loves Rosa. And this puts a dent in their relationship. Kain isn't honest with his feelings and tries to hide them from his best friend, allowing his jealousy to simmer in the back part of his mind. While this keeps the awkward at bay for a while, when someone with mind-control powers strolls into town, it makes Kain an easy target.
Cecil is mostly in disbelief about Kain's betrayal, and tells himself and Kain that it's all the result of mind control. It is, to a degree, but it's not nice to admit your repressed love for your best friends ex may be having some negative reprecussions on your relationship. Cecil is steadfastly loyal to Kain. While everyone in the party, bar Rosa, has negative things to say about him, Cecil's first and only concern is getting his friend back.
When Kain returns to the party, Cecil is quick to forgive and speaks of his betrayals no more. The same is true when it happens a second time. Cecil doesn't care about the betrayals, he just wants his best friend back.
At the end of the game, Kain goes to seek forgiveness on Mount Ordeals. Cecil and Rosa have already forgiven him.