Go to the start of the scene where Kirika uses the popcorn to find her opponents then press Left, Left, Right for a hidden interview.
Nude Kirika
Enter the "Extras" menu. Highlight the "Clean Closing Animation" option, then press Left, Down, Left, Down, Enter to see a nude Kirika sketch.
Nude Mirielle
Select the "Chapter Selection" option at the main menu. Wait for the sequence of the man getting shot, then press Up, Down, Up, Down to see a nude Mirielle sketch.