Here is the full list of all 20 HuniePop achievements. It takes around 30-35 hours to unlock all of the achievements on Windows.
Spend the night with ALL the girls within 18 days.
Spend the night with ALL the girls without talking with any of them.
Spend the night with 3 girls without upgrading any of your traits.
Earn a 100% completion rate.
Successfully complete 10 dates while in Alpha Mode.
Successfully complete 5 dates while in Alpha Mode.
Give Kyu ALL the pairs of panties after she asks for them.
Spend the night with ALL the girls without failing a single date.
Spend the night with ALL the girls without giving any of them an alcoholic drink.
Unlock the secret character Celeste.
Successfully complete a date with at least 20 Moves remaining.
Unlock the secret character Momo.
Spend the night with the goddess of love.
Unlock the secret character Venus.
While on a date, earn 2000 or more Affection in a single move.
Spend the night with a girl within 4 days.
Max out both your Munie and Hunie.
Unlock the secret character Kyu.
Spend the night with a girl for the first time.
Have your first successful date with a girl.