Characters are one of the most important part of your book, without them no story can go on. Characters are not only defined into humans, but also fantasy hybrids, cats, dogs, reptiles, animals, inanimate objects, etc. Everything mentioned in the story can be a major key to influence the story right then or later in future.

A common mistake a lot of young or beginner writers do is make unrealistic Characters, yes, if the genre is fantasy then it is bound to have unrealistic parts in it, but what I'm focusing on right now is their psychology.