Here is the full list of all 30 Lady in Mystery achievements.
You've seen all 23 of episodes.
You've collected all the endings including bad endings.
You've seen the true ending of Seoyeon.
You've sold Seoyeon's gift.
Seoyeon gave you a Seonchu as a gift.
You've learned Seoyeon's heart of hearts.
You've seen Seoyeon feeling seasick.
You've seen the true ending of Iljime.
You've found Kang Yurin's secret storage.
You've seen Iljime's wound.
You've made Kang Yurin furious.
You've been gifted a Fire Bead from Iljime.
You've seen the true ending of Juhee.
You've died more than 5 times.
You've found a strand of hair in Pyeongho's drawer.
You've chosen Juhee over 'Revenge.'
You've seen the true ending of the revenge route.
Meet the guru at the fortune teller's street.
You've tried to suppress Juhee's compulsion.
You've met Heesoo in women's clothes.
You've seen the true ending of Yurin.
A servant girl's love has been exposed.
Kang Yurin came in to your room.
You've seduced a woman in afterlife.
You've cleared all the common episodes and now reached the the point where you can choose your own episode.
Heesoo has sung a song greatly.
You've asked the pale worker on episode 4 why he can't look at the blood.
Arrest the suspect on episode 2.
You've met the one who can change your destiny.
You've answered more than 3 times with the word that didn't exist in that time period.