They are incredibly rare fish, and are much more difficult to catch than other fish in the game. To see requirements for each fish, check the table below. You must catch all 6 King Fish if you wish to marry Kappa in the female version.
You will not be able to catch any of the King Fish without a level 6 or higher fishing rod. This means you must have a cursed/blessed or mythic fishing rod.
Each fish has it's own special requirements, which is listed in the table below.
List of Fish
Have Sushi, Sashimi and Grilled Fish recipes in your Recipe Book.
Can only be caught between 10PM and 8AM. Can only be caught in Spring and Winter.
Can only be caught in Winter.
Ship 200 fish. Cannot be caught in Winter.
Catch the other 5 King Fish. Fish in the pond in the 9th floor of the Mine.
Throw a smaller fish into the ocean before fishing. Cannot be caught in Winter.