Here is the full list of all 6 Creature Romances: Kokonoe Kokoro achievements. It takes around 0-1 hour to unlock all of the achievements on Windows.
Completed Day 1 in Kokonoe Kokoro
Completed Day 2 in Kokonoe Kokoro
Completed Day 3 in Kokonoe Kokoro
Completed Day 4 in Kokonoe Kokoro
Completed Kokonoe Kokoro
Completed any bad ending in Kokonoe Kokoro