EastarJet - airolines review - flight ZE602 from Tokyo to Seoul
Air connection between Tokyo Narita and Seoul Incheon can get very expensive. One way flight on kayak.com is about $400. EastarJet offers cheap alternative but at the moment information provided is only very ambiguos.
(this is probably first English blog artictle on flying Eastar jet. I hope it'll help you plan your trip.)
One would think there must be bunch of low-cost airline flying this route between these two major airports. But surprisingly there is only one. Flight ZE602 and ZE601 with EastarJet. In Asia, lowcost flights market is very different to the European one.
Airlines Eastarjet looked a bit suspicious at first moment - they don't even offer option to buy flight from Japan to Korea in English(only in Japanese). There was no chance to get any community information on the internet in English. And also the website tells you, you won't be boarded if you won't have round trip.
Despite that I risked buying one-way flight from Narita to Incheon. Like this I saved around $250 - the flight costs proximately $150 - it is not as good bargain as you can get in European low-cost market(where Ryanair pushes prices pretty low) , but it is affordable.
As I wrote above for flying from Narita to Seoul you'll need to make the reservation in Japanese version of their website. It is not a big problem if you use Google Translate or use as a hand the English version where form is translated. Or at last you can call them(but you will have to be able to transfer money to Korean account in Korean Won). This could have changed since I wrote this article.
Though you will be probably a bit curiosity as the most passangers are Korean you will get as good service as possible. It is Asian standard. The flight is served by older Boeing 737-800. Checkin of luggage is already included in price (what can surprise many European travelers). The stewards/stewardess were super nice, all spoke fine English and noone even asked me for the return flight... So I definitely recommend.