Here is the full list of all 26 Eternights achievements.
Earned all trophies.
Date every dateable character.
Reach max rank with all your confidants.
You went on all of Yuna's dates.
You went on all of Sia's dates.
You made a promise with Sia.
You went on all of Yohan's dates.
You made a promise with Yohan.
You went on all of Chani's dates.
You went on all of Min's dates.
You made a promise with Min.
You are a social butterfly.
Going out with 1 person only til the end.
You retrieved the Stone.
Reached max rank with yourself.
Reach max rank with a Confidant.
You made a promise with Yuna.
You helped Min break through the second wall.
You scavenged 10 city items.
Now you know how to give proper piggyback rides.
You helped Yuna destroy the first wall.
You got creative with your hand.
You explored the city for the first time.
The start of a peculiar bond.
Escaped the shelter.
You awakened your new arm.