Bokutachi wa shitte shimatta
by Miyasaka Kaho and Takase Yukano
translated into french by Sumire no Team
Re-translated into spanish by Pig rabbit fansub
the story takes place when Kotori was still in middle school.
You will finally discover Kotori's unrevealed secret!!
Progress:
chapter 1- released
chapter 2- scanned
chapter 3- scanned