The trout/char species in Japan are small. The trout are called "Yamame" and the char (related to our Brookie) are call "Iwana". I landed several "reasonably sized" Yamame; but they were only 8" long; sometimes smaller. They're thin, move like bullets, are very easy to spook and live in the almost gin-clear water of the mountain freestone creeks. The flies that they use are, appropriately, small in size; a #16 would be a big fly for Japanese trout/char fishing.
I found that the good ole parachute Adams pattern was a match for anything that the local anglers were tying onto their tippet. I later sent my fishing buddy, Hideto-san, a fly box filled with just one fly; para Adams in sizes #18-22. They were greatly appreciated! And, more importantly, they produced fish for him.
If I wanted to spice up a fly box for a Japanese angler, in addition to the para Adams, I would add a para light cahill (I know I saw some mayflies that looked like light cahills, but I didn't have a fly that matched with me at the time), a BWO pattern and a sulfur pattern; all in sizes #18-#22. Nymphs also work well; particularly in the roughwater riffles. For nymphs, I'd stock in some Princes, and bead head pheasant tails/ hare's ears. All in the same small sizes' #18-22.
That would be it for me. Hope that you're able to put together a box that your Japanese angling buddy will enjoy.