Originally Posted by The Man
well, the FF4 Foundation Guide has Cain and Rosa being an item way before the start of the game, before he even gets his Dragon Knight training. However, due to a combination of Cain's long absence and (possibly) the fact that Cecil gets raised to the more powerful Dark Knighthood, she leaves him for Cecil. Due to this reason and a number of others, I dislike Rosa somewhat, since arguably, Cain wouldn't have been controllable by Golbeza if he hadn't had residual feelings for her. I don't loathe her as much as I used to, however, since I can kind of identify with getting lonely after a long separation; I'm more inclined to loathe the standard of a monogamous relationship that permeates pretty much all parts of society.
At any rate, the FF4 Foundation Guide was never released in English, but it was published by Square, apparently at around the time the game was released, so it's canon. It's also the source of things like the characters' last names and their heights and weights, which you can find all over the Internets (I even had that stuff on my old, old webpage before Comcast decided to 403 it).