Matsuya, Speedy and Filling
It's a bit difficult to define "fast food" in Japan in my opinion. Is fast food a meal that is prepared really fast? Or is it take out? or does it generally refer to junk food? I would say that fast food is cheap, tasty, not necessarily the healthiest, and can either be take out or dine in.
Matsuya has got to be one of the quickest prepared meals money can buy in Tokyo. The quality is astonishingly high for the price, and the taste is signature, and tends to stay delicious and filling even if you're a regular. The main item on the menu is called "Gyu-don" which is often called a "beef bowl" in English. Just simple, white rice topped with stir fried beef and onions, with a side of miso soup to wash it down and balance your palate.
You have a few options for free toppings at Matsuya also, but the safest bet is the red pepper, and strips of pickled ginger (the pink things in the picture). With the standard size at about 350 yen, you may find yourself stopping in a Matsuya for lunch pretty often.