Unlocked by completing Asa's route. Asa's route does not require any other routes to be completed first, so it can be done on your first playthrough.
Who will you go with?
- Wait for 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?
- Say that it's your turn.
Who would you like to go with?
- Go with Asa.
Who do you choose?
- Choose Asa.
It's worth mentioning that all the choices (with the exception of the last one) are also required for her twin sister Yoru's route. However, Yoru's route cannot be done until Asa's has been completed.