You can get Ageha's route your first time through, with no special requirements. You will need to make certain choices when the game prompts in order to end up with Ageha:
Who will you go with?
-Wait for Ageha.
Do you like Amane?
-Choose "No, that's wrong."
Who's swimsuit do you like best?
- Choose Ageha.
Who's turn is it tonight?
- Choose Ageha.
Who would you like to go with?
- Go with Ageha.
All of these choices will allow you to end up with Ageha at the game's finale, securing her ending and gallery photos.