These are all the islanders living on Monster Island currently. More will be added as the story goes on.
(Named from left to right from the bottom up)
Filthy Frank: A YouTube celebrity known for being... well, filthy.
Mothra: The Goddess Guardian of Earth from Godzilla Warriors.
Rebecca: My IRL friend, and Will's IRL girlfriend.
Kyle: Another IRL friend.
Sam: Myself. I'm pretty awesome.
Will: Another IRL friend, and Rebecca's IRL boyfriend.
Godzilla: The heroic atomic Kaiju from Godzilla Warriors.
Waluigi: A previous resident of Vineland Island.
Anguirus: The hammer-wielding soldier from Godzilla Warriors.
Morty: The constantly nervous grandson of a drunken scientist from the show Rick and Morty.
Rick: A drunken scientist from the show Rick and Morty.
Tora: A young catgirl and Kiti's little sister. Created by user/TheWildZach.
Kiti: A catgirl and the older sister of Tora. Created by user/TheWildZach.
Rey: A young scavenger from Jakku from Star Wars: the Force Awakens.
Kylo Ren: A powerful Sith lord, and the enforcer of the First Order from Star Wars: the Force Awakens.
Rodan: The high-flying daredevil from Godzilla Warriors.
Peridot: A high tempered Gem engineer from the show Steven Universe.
Amethyst: The easygoing defective Gem soldier from the show Steven Universe.
Sans: A skeleton with strange powers, and the brother of Papyrus from Undertale.
Steven: A human/Gem hybrid child from the show Steven Universe.
Yahtzee: A popular internet videogame reviewer.
Papyrus: A skeleton soldier-in-training, and the brother of Sans from Undertale.
Garnet: A powerful Gem fusion, and the de facto leader of the Crystal Gems from the show Steven Universe.
Pearl: A neurotic ex-servant Gem from the show Steven Universe.
If you want to see a character on Monster Island, feel free to leave a request in a comment. Not all requests will be taken, but if I like the character you suggest, I'll probably add them.