If you feel comfortable enough and are sure the other person is not a mass-murder, you can meet and see how it goes. One Japanese guy happened to live not too far from me and so we met. If you have difficulties finding a partner, I think they’re not too bad.
And if you’re looking specifically for a Japanese partner, but don’t live in Japan, then this might be a great option for you!
It is not limited to Japan, but you can set your profile so that you’ll find people living in Japan or with the same interests etc.
I’ve managed to get to know quite a few interesting people that way. In order to be able to exchange messages with others one of you needs to have a paid account, though.
Regular readers of my blog know that I’ve left Japan about a year ago.