There might be a castaway who has to adapt to the island life. There could also be a group of sailors who end up on the island by accident. Among them, there may be a leader figure who tries to keep everyone in order and find a way to get off the island. Additionally, there could be a local on the island who helps or hinders the outsiders depending on the story's direction.
In 'an island novel', the main characters could include a scientist who is interested in the unique ecosystem of the island. There may be a journalist who wants to report on the island's secrets. And there could be a family who gets stranded on the island and has to work together to survive. Each character would have their own goals and personalities that interact and shape the story.
Without having read 'The Island' by Benchley, it's difficult to name the exact main characters. But it's common in novels set on islands to have a protagonist who is a bit of an outsider. Maybe someone who has come to the island to escape their past or in search of something. There could also be a character who is native to the island and acts as a guide or a source of knowledge about the island's secrets.
There could be immigration officers or officials at Ellis Island as main characters too. They might play a role in determining the fates of the immigrants, and the novel could explore their perspectives, like how they deal with the flood of new arrivals and the rules they have to enforce.
There could be a group of friends who decide to visit the island together. One of them might be the leader type, always making decisions. Another could be the comic relief, always cracking jokes even in difficult situations. And perhaps there's a more mysterious character whose past is somehow related to the island.
There could be a variety of characters. There might be a strong and resourceful man who is good at building shelters and finding food. There could also be a woman with a kind heart who takes care of the others emotionally, and perhaps a child who brings out the protective side of the other characters.
I'm not sure as I haven't read it. But usually in a novel, the main characters could be locals from the island, or outsiders who come to the island and get involved in the story in some significant way.
Since I don't know the exact 'monster island novel', I can't say for sure. But usually, there might be a brave hero or heroine who leads the exploration or survival efforts on the island.
In 'Shutter Island novel', the two most important characters are Teddy Daniels and Chuck Aule. Teddy is a complex character with a troubled past. He's a U.S. marshal who is determined to find out what happened to the missing patient. Chuck Aule is his sidekick, more or less. He's there to support Teddy in the investigation, but as the story progresses, we see that their relationship and their individual roles are more nuanced than they first appear.
There's Cyrus Harding. He is a very intelligent and resourceful man. His knowledge in various fields like engineering and science helps the group a great deal. Then there's Gideon Spilett, a journalist who is observant and can record their experiences well. Also, Pencroff, a sailor with practical skills such as building and sailing, which are useful on the island.
I'm not sure specifically as I haven't read the exact 'Star Island novel'. But usually, in a novel set on an island, there could be a protagonist who is a young and adventurous person, maybe someone who is trying to escape from their past life.
There might be a character named Ralph. He is more of a thinker and planner, and helps the group make decisions about things like building shelters and finding food. He is good at coordinating with others.