You have to help a fisherman to capture the Kappa using a Rotten Fish you found. To catch it you must be clever because the Kappa is sly and sneaky. It will try to use different routes to get the fish so as to avoid capture. You must block all the routes so the Kappa will run straight into your net and capture him.
It likes to eat fish, even rotten ones.
However, in mythology the Kappa is supposed to love the taste of cucumbers and human blood.