Imps are creatures that can disguise themselves into other people, based on what they see. There are two types of Imps. Ones which the Demon Warlock controls who are faithfully loyal to him, or Imps that the Demon Warlock created, but ended up having a mind of their own and wanting to live normal human lives.
They usually disguise as children. If the person they turn into is wearing a hood or such, for example Aaron, they would only be able to copy what they saw, so if the "person" had to take of their hood, they would probably look completely different.
Imps are a reddish color, small with white horns and wings.They have yellow eyes and large noses that stick out of their faces. They are also roughly the same size as a young child, like Dmitri and Nekoette, but are able to expand their size to look as tall as a regular minecraft person, like Jeffory.
Imps are usually very loyal to the Demon Warlock, but some had a mind of their own and wanted to live normal human lives. So if you see the same person twice on the island, it could be an Imp or the real person. Imps tend to have extremely bad social skills, so it would be really weird to try to talk to them.