Also known as perilla, it's a close relative of basil and mint and shares some of their sweet, eucalyptus-like aromas. Green shiso is commonly served with sashimi or sushi or can be chopped and stirred into cold dishes. It's rarely cooked or used in hot dishes, with the exception of being battered and fried as tempura (often stuffed!). Red shiso is the main flavor in umeboshi, the salty purple pickled plums you'll find served with rice or stuffed into onigiri. It's also sold dehydrated and mixed with salt as a condiment for rice.
Here's a bomb for you: you have probably never tasted wasabi. The vast (and by vast, I mean vast) majority of wasabi served with sushi, sashimi, or soba in both the U.S. and in Japan is not wasabi at all. Rather, it's powdered horseradish mixed with green dye, and perhaps a bit of daikon radish for flavor and texture. Real wasabi is related to horseradish, but has a sweeter, milder, fruitier flavor.
For best flavor, it should be ground immediately before using, most traditionally on a wooden board lined with rough sharkskin. It used to be nearly impossible to get in the U.S., but these days there are a few farms in both California and Oregon that have started selling it. It runs around $100/pound, but you don't need much. This little nub cost $17 and is enough to feed a small army of sashimi-eaters.
Known in Japan as the "King of Mushrooms," maitake are one of the most commonly consumed fungi in the country. In the U.S. they're known as hen-of-the-woods or sheep's head. They boast a deep, earthy, savory flavor and are best when roasted, seared, or grilled in large chunks, though they also make a good addition to simmered soups.
Small, finger-sized peppers that are a common bar snack in izakaya all across japan. They're served whole, either simply grilled, deep fried, or stewed in a soy-based sauce. They have a distinctly grassy, mildly bitter flavor not dissimilar to a green bell pepper. The fun comes with the fact that about one in ten of them is hot, and I know of no reliable way of telling which it'll be until it's already in your mouth. It's like the Russian roulette of the Japanese bar snack world.
Not the most commonly used of Japanese mushrooms (they are outranked by maitake, shishito, enoki, and shimeji), nonetheless nameko deserve a spot for their uniqueness. Commonly used in miso soup or other simmered dishes, nameko has a slightly slimy, mucilaginous coating that can take some getting used to, but once you do, you'll appreciate its mild, nutty flavor. The Japanese have a thing for slippery foods. This is one of them.
Nagaimo or Yamaimo (Mountain Yam)
It's the only yam that is edible raw, and is mostly consumed that way. It's prepared by briefly soaking it in acid to remove some minor irritants, then gets grated into a slimy, mucus-like paste (I told you the Japanese have a thing for slippery foods). The paste is bland—almost flavorless—but is prized for the texture it adds to noodles or rice in dishes like tororo (cold noodles with grated yam).
The flowering buds of a plant, myoga tastes somewhat like a milder form of ginger, with crisp, crunchy texture. It's used mainly as a finely sliced condiment for soups or pickles, though it can also be sliced and sprinkled onto roasted meats and vegetables.
Also known as Japanese Parsley, it has a distinct celery-like flavor with tender parsley-like leaves and stalks with a crisp, clean crunch. The leaves and small shoots are mainly used in salads and as a flavoring for clear soups. You'll occasionally find it used as a garnish for sushi or sashimi as well.
An indispensable ingredient in Japanese cookery, daikon is used in a multitude of ways. Sliced into disks and simmered in stock for soups, salted and made into dozens of varieties of pickles, grated and mixed with ponzu to form a dipping sauce for tempura or noodles, sprouted and used for salads and sashimi, the list goes on. It's unlikely you'll eat a single meal in japan without daikon appearing in some form or another.