Shiawase is a Japanese restaurant in the south of Paris, near Pernety métro station (line 13). It's lively and cheaper than Montparnasse, if you with friends one night you'll find it very nice. It's discrete with a facade that's barely noticeable. They serve sushi, various menus of Japanese cuisine, the fish is very fresh and it's cheaper than the other Japanese restaurant next door. They are friendly and sometimes offer a digestif/snack.
Kamogawa is a Japanese restaurant on Hauteville street in northern Paris, adjacent to Franz Liszt Square where you can go for a drink after. It is a single room with capacity for about 30 people, with Asian decor and discrete. It's a pretty good place is open every day until 11 pm, it closes 3-7 pm. The menu is 8-12 euros with a main dish and a soup. It's pretty cheap and there's much to choose from. Sushi, maki, some vegetable tempura and fried makis. The Kamogawa is packed at noon though at night there are less people. Nice and quick service.