Here is the full list of all 12 UNDRESS! achievements.
Watch a scene 1 with Naoka
Watch a scene 2 with Naoka
Watch a scene 3 with Naoka
Watch a scene 1 with Sayri
Watch a scene 2 with Sayri
Watch a scene 3 with Sayri
Watch a scene 1 with Chizuka
Watch a scene 2 with Chizuka
Watch a scene 3 with Chizuka
Watch a scene 1 with Yuka and Yuki
Watch a scene 2 with Yuka and Yuki
Watch a scene 3 with Yuka and Yuki