You have to collect 5 mementos in "THE EMPTY SEVEN'S STORY".
1. Before heading down the lift that becomes available to use after killing all the android ninjas, the memento is in the back right corner. You should see it during the enemy encounter, but cant reach it until the force field is dropped, by killing all the enemies.
For 2-5, it will depend on which lever you pull first which will result in a stone coming out of the wall, allowing you to reach the area, there are three spots, I started with the left one, then did the middle, then the right one, so my info will come from doing it in this order.
2. (Left Wall Area) Before entering the Boss area, go to the right, and at the end of the walkway will be the memento.
3. (Middle Wall Area) On the ledge that has the Otsuru Bear, continue over the edge and drop down and the memento is right there on the ledge.
4. (Right Wall Area) The memento is located on the first ledge after jumping up the initial rock that comes out of the wall, once up on the first ledge head left to the furthest most ledge, and there will be the memento.
5. (Right Wall Area) When facing the Boss Chamber door head to the left and the memento is at the end of the path.