The Kappa is a magical being who lives in the lake on Mother's Hill. He will appear after you throw a Cucumber into the water. Kappa doesn't talk much, but he'll accept your offering and then return underwater. He will only accept Cucumber; the game won't allow you to throw anything else into the lake. What a strange fellow...
Birthday: Spring 8 (primary) or Spring 9 (alternative)
Bonus LP: None
Schedule: Kappa doesn't leave the comfort of his water. Because the lake's water freezes during Winter, you can only give cucumbers to Kappa during Spring, Summer, and Fall.
Kappa will not live inside the farmhouse if you marry him. His forever home is Mother's Lake. The child that will be gifted to you will be a normal human.
+800 LP (+9 FP)
+500 LP (+9 FP)
+300 LP (+3 FP)
-500 LP (-3 FP)
-800 LP (-9 FP)
Kappa was a challenge candidate added to Harvest Moon: More Friends of Mineral Town, similarly to the Harvest Goddess in the boy version of the GBA game. Kappa has the same marriage requirements in the Switch remake, but with one requirement that if not met, will completely block you from marry Kappa.
Throwing 20 gifts into the Goddess' pond (one gift per day) will prompt her to ask if you have your eye on someone special. She will then present you with a list of all marriage candidates. You must pick Kappa from this list. Picking anyone else from this list will prevent you from every marrying Kappa.
Since there is a 10% chance of losing 200 LP if you don't toss a cucumber at Kappa every day, you may find that his Love Points will slowly decrease during Winter. This is easy to recover from when Spring arrives and you can start throwing cucumbers into the lake.
Just like with the normal candidates, Kappa requires a Big Bed inside of your farm house. He will never use this bed though, as Kappa does not live inside of your house once married. You'll also need to have 60,000 LP (red heart color) with him.
The additional requirements you'll need to meet are:
You need to be in year 5 or later. Simple enough!
Complete Shipped Items List
You must ship at least 1 of every item in the game that can be shipped through Zack's shipping bin on your farm. This includes more than what is noted on the shipped items list in your farmhouse bookshelf. You'll also need to ship a weed, fish bones, Relax Tea Leaves, and every other item that can be tossed into the shipping bin.
Catch All Fish Species
You don't need to catch the largest of each fish, just that you catch all the fish. This includes the Whopper fish.
Find All Mine Items
Confess Your Intent
On Mondays, Wednesdays, or rainy/snowy days, Carter will spend time in his confession room inside the church. You can chose to ask him to unequip a cursed tools, confess something you feel guilty about, or state you're just there to chit chat.
For completing this marriage requirement you'll have to confess to Carter. This choice will give you three random pleasures that you are expected to feel guilty about. One of those random options is admitting you want to become partners with the Kappa. Select that option and Carter will give you his opinion on the matter. He'll either tell you that it isn't going to happen or he'll give his blessing for your courtship with the lake dweller. You'll need this blessing for your marriage.
Collect the Kappa Orbs
In the game there are 9 round orbs hidden in the rocks within the Spring Mine on specific floors. When discovered, the orb will appear on the ground and then absorbed into the item section of your rucksack; you can find the orbs even if your bag is full at the time they appear. There is only one orb per specified floor and it will not reappear on that floor once you've discovered it.
The Kappa Orbs will appear on floor 0 (the entrance), 40, 60, 80, 120, 140, 160, 180, and 255.
The game will NOT keep track of which floors you found a Kappa Orb on. Keep note of the floors outside of the game so you don't end up forgetting which floor you collected an orb from.
Once you discover the last orb, it will immediately merge with the other orbs (regardless if the orbs are in your rucksack or your farm house storage shelf). The resulting item will be the Kappa Gem, an accessory that can be equipped to recharge your Fatigue when you are outside.
Once you've completed all of the marriage criteria, you can propose by tossing a Blue Feather into the lake. If he accepts your proposal, 7 days later you'll have a private ceremony at the church with the Nature Sprites as the ceremony guests. If he rejects your proposal, then the requirements have not been completed.