I think there’s another requirement for marrying the Kappa (http://www.fogu.com/hm5/index.php?p=boys/kappa ):
You have to give 20 gifts to the Harvest Goddess. When she asks you if you like a boy you have to choose Kappa.
The reason why I think this is another requirement:
A few years ago I tried to marry Kappa but I wasn’t successful: (http://fogu.com/hmforum/viewtopic.php?f=6&t=34186 )
I started a new game. At first I made all the requirements. Then I started to give him a wrapped cucumber every day (beginning in spring). When the blue feather appeared in the supermarket (after ~50 wrapped cucumbers) I knew that Kappa's heart was orange (in my first game a few years ago I had already bought the blue feather when Kai's heart was orange in year 1. So I never knew when Kappa's heart was orange). 25 wrapped cucumbers later he still said "no". But his heart must have been red (I needed 50 cucumbers from black heart to orange heart, so 25 cucumbers must be enough for orange->red heart).
Then I rembered about the H.G. and gave her a few gifts (I had given her several gifts before so I only needed a few days). When she asked me about my favourite boy I answered "Kappa". Then I showed him the blue feather and I could marry him.
I’d saved before the H.G. asked me if I liked a boy. I reloaded and gave him a wrapped cucumber every day until the end of fall.The next year (spring 1st) I tried again to marry him (Once more I told the H.G. that I liked the Kappa) and I was successful. I reloaded and tried to marry him without telling the H.G. that I like the Kappa and again I wasn’t successful. I gave him a cucumber every day for 2 more months and tried to marry him again on summer 30th. I was only successful when I told the H.G. that I like the Kappa. So I think it’s a requirement for marrying him
