A Complicated Romance
The love story in Final Fantasy IV is an interesting one, as it's not one sided or simply just reciprocated in two parties. Instead, there's a love triangle between a protagonist and semi-antagonist.
Rosa, Cecil and Kain were childhood friends, growing up in Baron together. Albeit with a rocky start, Cecil and Kain became rather close friends. Over time, both Cecil and Kain had developed romantic feelings toward Rosa. She, on the other hand, had taken more interest in Cecil than Kain in terms of Romance.
“They told me you'd died in the earthquake at Mist. But...I knew it couldn't be true.”
As for Cecil, while at the beginning of the story, he thinks highly of Rosa and appreciates her, he is unsure of pursuing those interests any further. For one, it could be the Dark Knight which is gradually causing his heart to darken. On the other hand it could just be general apprehension or not wanting to see any danger come to her. Rosa is sure of herself and her capabilities as a white mage. She is persistent and refuses to back down at any point. At the same time, she never forces the issue of love or affections on him. Yet she openly lets him know how much he means to her.
It's likely that Rosa felt closer to Cecil from the beginning. At their first meeting as children, Kain threatens and attacks the young Cecil for the attention that he receives from the king. Rosa runs to his aid and gives him a good tongue-lashing, while Cecil admits that it was his fault.
Kain on the other hand, has become jealous of the attention that Cecil is receiving from Rosa. It's likely that this jealousy is long-stemmed even from childhood, not only with Rosa's involvement but the King's interest... having no idea that Cecil was a Lunarian descendant. His jealousy and feelings toward Rosa become obvious during the time when Golbez kidnaps Rosa.
Rosa runs into the crystal chamber just as he's ready to finish of Cecil. She yells at him to stop. At which point, he holds his head, the spell's grip on him seems to be diminished as his emotions return. At his point of weakness, Golbez enters the room and commands Kain to take the crystal. Rosa continues pleading with him to stop despite the danger in doing so. At that point, she is taken by Golbez. While captured, Kain tells Rosa what he thinks of him.
“I am more than Cecil will ever be. You'll see that soon enough.”
Having become a Paladin, casting off his dark armor, Cecils heart begins to open. They say that “absence makes the heart grow fonder”. It really is true in this case. Cecil rescues Rosa in just the nick of time, grabbing her seconds before a guillotine drops above where she was bound. He embraces her, realizing what she meant to him.
“I realized the moment you left my side. Rosa, I-”
Kain, having been released from the spell realizes his wrong doing. I think at that point, having betrayed Cecil and the others and putting Rosa into harm's way weighed more heavily on his heart than his own personal feelings. He sticks with the party for the rest of the journey in the hope that he can somehow make amends.
“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.”
After the defeat of Zeromus, everyone returns home and to their own respective places. Rosa and Cecil return to Baron. Kain instead goes to Mount Ordeals in order to become a better Dragoon and ease the personal feeling of guilt from his betrayal. Cecil is crowned king of Baron, and Rosa becomes his queen. The two are happily married at the very end, overcoming a great deal of hardships.
