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