I agree that all four parts are basically equal -- I think that Sky and Sarah have more depth as characters, but in terms of stage time and importance to the story, they're all pretty much equal. There are arguments for either couple being "the leads" -- my director gave us (Sky and Sarah) first billing and last bow, but it could go either way. If one
character had to be called the "catalyst," it would be Nathan, I think (not Nathan and Adelaide, just Nathan), because its his need for the $1000 that sets off the whole Sky and Sarah plot, like Jordan said. "Catalyst" isn't necessarily "lead," but it does mean that Nathan might have a slight edge on the other three as to how important he is to the story.