An otherworldly being. This new character could come from a dimension that is connected to the 'the other side of darkness'. They have unique powers and abilities that are completely foreign to the characters in the original story. Their arrival could disrupt the balance of power and force the existing characters to reevaluate their understanding of the world.
A young and curious apprentice. This character is eager to learn about the mysteries of the world in the story. They might accidentally stumble upon a major plot point while exploring on their own. Their inexperience could also lead to some comical and dangerous situations that the other characters have to bail them out of.