Recently we discovered that Kawamatsu is the mysterious prisoner that was first shown in Chapter 924.
Kawamatsu was seen in chapter 936 to have the laugh “Ka pa pa pa”.
Kawamatsu may have the Mythical Zoan Kappa Devil Fruit.
Kappas are mythical river monster creatures that eat fish.
This explains why Kawamatsu eats fish and maybe even tolerates poisonous fish.
Kappas love to engage in sumo wrestling apparently.. and there is this
Sumo Inferno Tournament currently happening in the Udon Prison.
Kappa is a portmanteaux of the words “kawa” (river) & “warawa” (child) – KAWAmatsu –
Kawamatsu stated that he has sea prism cuffs on so he is clearly a devil fruit user.
Additionally, people believe Kawamatsu was originally with Nekomamushi and Inuarashi when they first came to Wano as kids, leading people to think Kawamatsu is also a mink.
Kawaso is another work for kappa and it means “otter”.
There are also ties to Kappa being related to “River Pine Tree” being related to being an Otter in Japanese Folklore.
Oda has been packing Japanese folklore and mythical creatures into the Wano arc so far and I believe he will continue.
Kappa creatures come up in tons of Japanese children’s songs and Japanese folklore.
Kawamatsu may be an otter mink who ate the Mythical Zoan Kappa Devil Fruit.
*Theory by OnePieceYo