Niohplayers all know and love Kodama, the little green spirits lurking around the world in easily-overlooked places, but in Nioh 2, you'll also want to keep an eye out for Sudama, the purple fellas with skull masks. You aren't meant to round them up. They don't need your help getting back home!
Rather, you're meant to trade with the Sudama. The game isn't immediately clear about this – it just says "it seems as though it wants something" when you try to interact with a Sudama – but they'll take your unwanted gear and give you something potentially great in return if you're lucky.
The next time you find a Sudama (and you've cleared out all the nearby yokai), bring up your equipment menu and drop a piece of gear you don't plan on using – preferably something as high-level as possible. Drop the gear with the Triangle button and the Sudama will scurry over, take it, and give something else back. You might get a weapon, accessory, or armor piece that better suits your build.
There's no guarantee you'll like the trade – different Sudama like different stuff, and RNG can be wonky in frustrating ways – but I've been fortunate. I've traded weapon types I'm never going to touch and have gotten back a roughly equivalent version of my go-to, the kusarigama. You might get materials.
The lore is great, too. Sudama are actually Kodama who were corrupted by the Dark Realm.
"One who presents a Sudama with something it despises will incur its monumental wrath and invite punishment for their transgression. To lose one's life in this way would sadly and surely be some cosmic joke. As with all other things in life, pranks on a Sudama should be carried out in moderation."