Just because you're traveling in Japan, doesn't mean you can't experience some other flavors. In fact, I'd actually encourage it. While this is very easy to do in a big city, you may struggle in finding good non-Japanese options in the rural cities and areas of Japan. In Kochi City, there is one hidden jewel that you will have to check out.
Warung Cafe is an excellent, non-pretentious Thai restaurant that is tucked away in some back streets. Eating at Warung gives you the feeling that you may in fact be in Thailand having a home cooked meal at a friend's house—actually, the cafe is run out of the front of the woman's home, so that's not so far from the truth.
While this cafe is fairly well known by people in Kochi, it is not a place you would just stumble upon. And even if you did, it is not immediately obvious that it's a cafe. As I said, it's run out of the front of a home.
The space is very small, but the front is all opened up, so there is also some seating outside on the street. This, combined with the hodgepodge of tables, chairs and other décor, is probably what creates the sense that you might just in fact be in Thailand.
I recommend that you try to avoid the main lunch hours by going in just a little before noon on weekdays. It will fill up soon, and the wait can be fairly long as it's usually just one woman doing all the preparations.
As far as the food goes, it is very authentic and nice, but some of the items definitely turn up the heat, so be careful if you're sensitive. The tom yum soup is delicious and has just the right balance of herbs and spices. I would recommend any of the curries, as well as the nama haru maki (fresh spring rolls).
There are a few set menu items, but I usually just order one of the lunch options when I go. It's typically about 700 yen and includes a salad, soup, and the main course. She always has some Thai desserts out for about 200 yen as well, though, so that can be tempting. Fried bananas and peanut cakes are some of my favorites.
Warung is one of my top recommendations for places to eat in Kochi, and because it's not too far from Kochi Castle, I hope you decide to go by when you're in town.