The male protagonist is a Prince, heir to the throne of his kingdom. Rather, he was, up until the day his own family betrayed him. He was framed for something horrible enough to have such a right be stripped from him, so that his brother could take the throne instead.
Unable to prove his innocence, he was sentenced to exile or fled to avoid further punishment for the crime he never committed.
The young man had no intention of leaving things like this, however. He vowed he would take back all that rightfully belonged to him and make those who betrayed him regret ever doing so. Over time, carefully, he worked to be able to accomplish this, counting on the help of the few that still believed in him, as well as gathering new allies, people to fight for him and his claim.
The leading lady is...well, a Lady, of powerful noble lines (if not royalty), leaving her home to become marry the exiled Prince’s brother, current heir to the throne. The outcast Prince, however, now with allies are now greater in number and his forces having increased, has decided he will not allow this to happen, and thus kidnaps his brother’s bride. This is how the story starts.
As is probably obvious, this plot is fairly basic and open in terms of details, leaving a lot about the character you'd be playing up to you. (Though I'll be happy to help with decisions, of course.)
In terms of the Prince's family and betrayal, for example: In my first version, his father favored his brother and disliked him (because of liking his second wife, and thus the child she gave him, better), but certain laws/people wouldn't let him just make his younger son his heir, so he was also behind him being set up. It could also just be the work of his brother, though, with or without other people close to him helping.
His reasons to kidnapping the bride are somewhat open, as well.
They could have been friends/known each other before the whole mess, whether she was arranged to marry him or not, and he could want to keep his friend from marrying the "true monster" in this whole thing, despite her having been convinced that's his part.
Or it could instead be that, since she's arranged to marry the heir to the throne, he decides she is actually his and isn't about to let his brother have her.
It could also be some sort of statement against his enemies, or something that furthers his plans somehow.
It could be any of those, a mix, or something else!
Basically, you get to see and decide what kind of character you want to play, and basically give him his reasons accordingly. Just keep in mind I do want romance between the two, so as bitter, focused on revenge or such you make him (or not), don't make him a complete douche I (and my character) can't find any reason to like. xD