Japanese curry has a distinctive flavor profile quite unlike those of its Indian and Southeast Asian brethren. Home cooks throughout the world make it from packets of prepared spiced roux (to see why, check out this thread).
Here are some special additions that add extra dimension to your Japanese curry; try mixing one or more of them in to taste at the end of cooking: Bull Dog tonkatsu sauce; grated fresh apple; or maple syrup.
Some like to serve Bull-Dog on the side, so each diner can add it to taste. mochi mochi serves chopped peanuts and tsukemono (pickled vegetables) alongside.
Japanese Curry Tip