Here is the full list of all 8 Mandemon achievements.
Ayame or Kakitsubata?
The Renkai Academy has restored its rightful order. Seen the ending "Rebuilding."
You have firmly chosen to stand on the side of humanity. Seen the ending "Nihil sub sole novum."
Set off for the New World. Seen the ending "From the New World."
Who is Hayase? Seen the ending "Jekyll & Hyde."
Your existence is the meaning of life. Seen the ending "One must imagine Sisyphus happy."
You have ended this cycle, and your destination is the seaside. Seen the ending "Ending the Cycle."
Entered the special exhibition about Rachmaninoff.