-Every boy has TWO stats you need to raise in order to get his romance end. His primary stat needs to be at least 80, and his secondary stat needs to be at least 60. The second stat for each boy is the stat you raise when you spend time with him, so that should come fairly naturally.
-The snowstorm scene in the middle of the game is a good time to get a big boost in stats with the guy of your choice.
-The book you check out from the library will also increase your stats with a particular guy.
-The amount of money you earn has an effect on the epilogue, but not the end that you get. Simply put, if you do well, Susana will invite you back to work for her again.
Full walkthrough for all the four characters below: