Each girl requires you to have a certain amount of relationship with you before you can dive into a level of her cosmosphere. In general, you will only need a talk topic in each level of her sets (1.1, 2.1, 3.1, etc), but sometimes you will be told you have to wait for a story event.
The following is designed to get the 100% trophies, but some scenes will be listed as optional. Although they may say optional, it's best to do all events listed in case they are required for 100%.
