This achievement will unlock once you complete the game, with Kotori as your final choice of love interest. In order to finish Kotori's route, here are the choices that must be made:
Who will you go with?
- Go with Kotori.
Do you like Amane?
- Answer "No, that's wrong."
Who's swimsuit is the best?
-Kotori.
Who's turn is it tonight?
-Kotori.
Who would you like to go with?
- Go with Kotori.
Pretty straight forward. Essentially focus on Kotori and Kotori only. Finishing Kotori's route is necessary to unlock Amane's route.