The walkthrough says "6. In order to actually start the romance route you need to choose the option 'Let her go home' in the first room of the 'Red Maiden'. This option appears twice!" so I assume if you don't do that, you'll automatically start the corruption path - but don't quote me on that
The other characters are available as you continue the Rachel path and are all self-explanatory (nothing too tough about getting into their pants, although some variables are involved from time to time, such as money) - you may quote me on this though
Since there are hints in the game now, I think you should look into those while going through the story so that you don't get lost