Check the lockers, preferably in apartments (where the abovementioned ninjas are). The enemy you seek will be there.
Once you beat the game (any ending) - press the "continue" button while in the main menu. It will check your save file, and since the game was beaten, main menu will change how it looks a bit. If it did - gallery is open.
There are 3 endings. Conditions for them are:
Paizuri addict: >30 paizuri.
Paizuri obsessed: 1-30 paizuri.
Paizuri bride: 0 paizuri.