You cannot complete Yoru's route without first completing her sister Asa's route. Yoru's route can only be done in a second play through or later.
Choices:
Who will you go with?
- Go with Ageha.
Do you like Amane?
- Answer "No, that's wrong."
Who's swimsuit is the best?
- Do not pick any of the girls.
Who's turn is it tonight?
- Select "Me".
Who would you like to go with?
- Go with Asa.
Who do you choose?
- Choose neither of them.
This will secure Yoru's ending (also known as the "twin ending), landing you this achievement and the gallery photos.