While exploring a sewer, Raiden will come across an enemy with a small boy. During the intermission sequence, the boy will shout "A ninja?! Cowabunga!", then say "Go ninja, go ninja, go" to Raiden, a reference to Vanilla Ice's "Ninja Rap" song from Teenage Mutant Ninja Turtles 2: The Secret of the Ooze.
Collect all items from boxes in an area, then press (Select) and enter (Customization). Once you exit the customization screen, all item boxes will be reset, allowing you to fully stock up on items. Repeat this as many times you wish.