The third essential element needed for a story is the character guide. This doesn't necessarily have to be a book on its own, but whatever character you make as a writer, it is useful to have a back-up for when you forget what their personality is, or what their age is. Characters are, just like the plot, the backbone of a story. If there's no protagonist or no antagonist, how will a story develop when no one's there to write about? You can write about a forest, or just a place, but fiction is normally about a living character. Whether this is a talking tree or castle or just a human, it has human traits. So, what most people do is make a decision about their protagonist and antagonist first.
The things that are necessary for a character are simple. You need a name. This can be a first name and a surname, but also second and third names, or only a first name. Then you might need an age. Furthermore, you need to describe their appearance. This can be a story, but also three sentences. Then you sometimes need family, or a weapon, or a pet. And last but not least, their personality.
What some writers do is base one of their characters on themselves. This is easier since you know how you would respond and you don’t have to take on the identity of the main character. Other characters might be based on friends or enemies. For example, J.K. Rowling’s character Dolores Umbridge is based on someone Rowling really doesn’t like.