An old man who used to be a captain of a high-speed boat that went to and from Rokkenjima.
At least among those still alive, he's certainly the person most familiar with matters concerning Rokkenjima.
The media hoped he might have learned details of the situation inside the Ushiromiya Family from various discussions with servants going to and from the island.
However, he never said anything, so they eventually began to forget this old man...
Theories
Kawabata was hired to secretly carry stuff to Rokkenjima
Any effort to smuggle things and people on the island would have to involve him in some form.
This includes the equipment Sayo used for the murders, and the fake Eva recovered afterward in Kuwadorian.
The Ushiromiya group is the main suspect.
Leads:
Kawabata was the only person to regularly make the roundtrip to Rokkenjima.
He knew about Kuwadorian because we know he would secretly bring stuff there on Kinzo's orders 19 years earlier.
Kawabata found the second Sakutarou after the tragedy
The object Ange sees on display in the room of Captain Kawabata's grandchild is another Sakutarou. It was plausibly taken to Rokkenjima by Rosa who was planning to give it to Maria, then the captain found it and brought it back after the storm.
Leads:
It explains why Ange and her imaginary friends are freaking out while the captain and Amakusa have no idea what she means.
It's the knowledge she will later use to get Beatrice out of her golden land. She knew he could be revived because she saw it unlike Maria and Beatrice.
It was in the room of a child.
He may or may not have the power to shoot lightning.
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