To get the Friend to the Kodama Achievement in Nioh 2 you will need to get your hands on a rare item called the Kodama Soul Core. This is easier said than done, as there appears to be a degree of RNG involved.
To start, you will need to find purple Kodama, called a Sudama. While our green Kodama friends look all cute and helpful, a purple Sudama certainly does not. You can find one in The Beast Born of Smoke and Flames, the game’s second mission area. As you move through the burning village, you will come to a short bridge. On the right, you will see two houses. The second house contains a ladder you can climb down, bringing you to a tunnel that you can follow to find the Sudama.
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You can offer an item to the Sudama, and they will give you another item in return. It is a good way to try to get value from items you just don’t want, or armor and weapons you have no plans to use.
If you give the Sudama a Soul Core, the have a chance to give you a Kodama Soul Core in return. This is the item you need to get to finish the Friend to the Kodama achievement. The Soul Core you give them will be gone, so make sure you do not give them one that you want to use a lot.
Some players have reported getting better drops with better items, so it might be a good idea to give the Sudama any Boss Soul Cores that you do not wish to use, to try and improve your odds of getting the Kodama Soul Core.