Here is the full list of all 27 Guardian of the future achievements.
The first step to saving the world
Sleep mode
So funny
Have you seen it?
Tangible connection
Well...
So scary
Who is it?
Bad or good?
You should have done this
You need more sleep
What other choice you got, huh?
Unfortunately, this is yours.
He will be gentle with you
The inevitable is near
Terrifying, aren't they?
The diary is in your hands!
The ritual
He showed you something
They're gone
You're here. And what's next?
For you, yes
Tamaki's ending
Yoshiro's ending
Yuki's ending
Arashi's ending
Neutral end