Uh… Actually, only Futoshi Akamine and his son Ryo is confirmed on Kugai island.
Chronologically speaking, the events of Keiji Sakagami, the 50 year old baseball coach who appear in volume 1 and in flashback in volume 3 happens before Our MC, Tomoki Orihara came to Mizuza island. So it's unlikely that Tomoki took Keiji with him.
David Goldberg only appears in volume 2, though as a "distraction" since Tomoki can't go back to the island for the night due to his schedule. He trained him for only a day, and he literally left him out on the streets, in the rain, hogtied.
It's not know what happen to him afterwards, but Tomoki does not do loose ends, so he might be on the island, or dealt with in some other way…
Ryogo Nakaima appears in volume 4 and briefly in volume 5, with Yuzo Koyanagi, Tomoki's aide knocking on his door after it was reveal that it was him that gave Zyunji Kaneshiro the evidence to blakmail Tomoki.
Yuzo overpowers Ryogo, ties him, drugs him up and starts disciplining him. In volume 5, he was briefly mentioned when Tomoki asks Yuzo about any loose ends, and that it's impossible for Ryogo to recover, and that most likely he'll find normal speech hard for quite a while.
Again, it's unknown if he has been moved to the island, but I don't think Tomoki is interested in a drugged up slave.
Mitsuyoshi "Gon" Ahagon first appeared in volume 3, and in volume 5. In volume 3, Zyunji was looking for some info and finds Gon fucking some whore and joins in, DPing her. In volume 5, we see Gon getting arrested by the prefecture police for drug possession…?
Seeing as he was "arrested", and had no direct contact with Tomoki, it's highly unlikely that he was moved to the island, due to lack of interest.
Zyunji Kaneshiro, Futoshi's subordinate, appears from volume 1 til volume 6. He's your typical crooked cop, was supposed to be on the main island, but due to poor performance, was reassigned to the backwaters of Mizuza. He hates it there.
Does the normal blackmail, extortion, protection bracket, and after seeing the evidence of Tomoki abducting Futoshi, he see it as his ticket out of Mizuza.
He was brought onto the island in volume 4 to "help" train Futoshi, but ends up helping by forcing Futoshi to give up his freedom to let Zyunji go. Zyunji's last acomplishment on the island was making Futoshi cum hands-free.
At the end of volume 6, Zyunji was warned to takes this a his second chance and not to do anything stupid or something might happen to his family.
Yuzo Koyanagi appears throughout the series, though we really get his backstory in volume 7, Tomoki, seeing what his father has in Yuzo, wanted his own good, loyal slave that money just can't buy.
Kenta Gotou, Ryo Akamine's friend, appears throughout the series, Tomoki only took notice of him in volume 5 when he ran in front of his car. Tomoki joked to Yuzo about how much fun it would be to fuck his little monkey ass.
It's unknown if how his story will continue to unfold, but with the series being planned for only 10 volumes, it's unlikely that we'll see much of him.
Ryo Akamine, Futoshi's only son, appears throughout the series, in volume 6, it was shown that Ryo received a mysterious text message that gave him pause.
At the end of volume 7, when Ryo appears on the island, it was shown that the text message was a picture of Futoshi sucking Tomoki's cock and speading his ass cheeks while cum drips from his asshole, next to the missing poster that he has been posting all around Mizuza. The message said don't tell anyone. If you want to help your father, follow my orders. From this day forward, you are my slave.
In volume 8, it was reveal that while Futoshi was being trained on the island by Tomoki, Ryo was busy follow Tomoki's orders and training himself.