I wrote a review of Dosirak Japanese Korean restaurant near Syntagma Square.
I actually liked this place better than the Japanese-owned Furin Kazan. If I should speak about authenticity, Furin Kazan probably is the better, but for the value-for-money factor and for the variety of menu items, my preference goes to Dosirak.
Only negative factor is that inside the restaurant it does not smell Japanese cuisine. But, if you are not Japanese, the probability is that you won't notice and it is negligible.