After reading chapter 935 and seeing that the guy in the dark cell is Kawamatsu, I have reread chapter 921 because that was the chapter where Kawamatsu, Denjiro, and Ashura Doji were first mentioned.
There was a theory that was saying that the guy in the dark cell was the third mink ruler. In chapter 920, you can see someone else next to Inuarashi here. People speculate that it is the third mink ruler.
If you look closely, the hidden figure is presumably wearing a straw hat and I think that this is Kawamatsu. The reason I think that Kawamatsu wears a straw hat is because when Kinemon first mentions the three guys that are worth a 100 men, one of the silhouettes show a guy that is wearing something that looks like a straw hat.
Now we know that Inuarashi and Nekomamushi are affiliated with the Roger Pirates through Oden. In the cover of chapter 631, we see Crocus drinking with a man with a straw hat.
Since crocus was a member of the Roger Pirates, it would make sense if he was drinking with a mink because they were together in the past. In chapter 920, in the last page you can see all the possible allies of Luffy in taking down Kaido and among these characters is a a man with a straw hat so it would make sense that this guy is Kawamatsu because he is one of the three strong people that Kinemon wants to find.
*Theory by Katakuri_BB