Please be aware that Seoul and Tokyo has multiple airports and that these results are for all airports serving the city.
The best day to arrive at Seoul is Wednesday and it would be best to avoid Tuesday, as prices are on average 68.97% higher than on Wednesday. (Average prices, based on 346 datapoints.)
The cheapest airline flying from Seoul to Tokyo is Jeju Air. They are 45% cheaper than Korean Air. (Average prices, based on 201 datapoints.)
The best time to fly is at around 21. (Average prices, based on 322 datapoints.)