So if you check out the walkthrough, you'll see that each character has two attributes. Each one is tied to Blue and Red choices. Think of blue as the more "gentlemanly/nice guy approach" while the red is more "rakish / opportunist".
Your choices are cumulative, so selecting a lot of blue will lead you down one path, selecting red will lead you down another. (If you replay the intro with Nanami, pick all blue, then replay and pick all red, and you'll see a different follow up scene).
So using Jane as an example, if you pick predominately blue, you'll be working to break through the bad girl routine and help her straighten her life out. You'll still get romance out of the deal, but it will be more heartfelt.
But if you pick mostly red, then you walk with Jane a little ways down her path to self destruction and ride her wave of chaos to some kind of end.
There will be points along the way where you can "switch tracks" so you aren't completely locked in to one path or another. So even if you are on one path, blue or red, there will be chances to switch over.