Take one room, put a shelf (5 seats), some cabinets and a refrigerator – you get half-Japanese deli shop, half-typical Tokyo street restaurant. I recommend booking by calling in advance or using take-away instead.
As soon as you sit down, you are welcomed with the politest manners by the Chef, a really nice Japanese woman, and her husband. You are given a basket to put your coat and purse in and you’re given the menu. You can choose “à la carte” or you can choose different formulas. Some of them are only for lunch. There are single courses, there are Bento boxes and there are “don” combos (rice and something else).
I strongly recommend ramen. That’s the hottest and richest noodle soup I’ve ever tried. I also suggest gyozas: those are raviolis stuffed with meat and greens and you can add soy sauce and hot pepper sesame oil.
If you’re a fan, you should have Unagi, grilled eel; they say it’s like having it in Japan. And you can’t leave Kokoroya if you haven’t tried onigiris: rice balls stuffed with tuna and mayo, or miso and spring onion. These look like they were taken straight out of an anime.
If you want you can also find single ingredients in the market inside the shop.
You can also find props for making bento, like containers and dividers.
I wouldn’t change Kokoroya for any of the all-you-can-eat sushi restaurants blooming every week in town.