Stock soy sauce, sugar, sake and mirin (which is just a sweet sake, so you can substitute sake and sugar) in your shelf all the time because these four ingredients are indispensable for Japanese cooking.
In Japan, we often use ginger too.
The Japanese cooking is most developed in seafood dishes. As for meat, it isn't really impressive, I mean technique-wise.
While it doesn't use so much oil, it uses a lot of salt. So be careful about your salt intake.