Amane's route is the shortest (in terms of questions) and can only be seen during a second play through or later. Finishing Kotori's route is required before you are able to have access to romancing Amane. Here are the choices that must be made:
Who will you go with?
- With Ageha.
Do you like Amane?
- Tell Amane that you like her.
Who's swimsuit is the best?
- Choose Amane.
Who's turn is it tonight?
- Amane's.
The "who would you like to go with" question does not show up in Amane's route, unlike the other four girls.. so don't worry if you think you've missed something. Choosing these options should secure this achievement, and help to unlock all of Amane's images in the gallery.