In theory, it doesn’t seem hard to establish who your main character is. After all, your main character is the main character because the reader spends the most time with him. If the story is in first person, he’s even the narrator, but at the very least, you probably get his thoughts and perspective, something you wouldn’t from other characters.
Or maybe your main character is the main character because all the stuff happens to her. She’s not just some random waitress who shows up once in the middle scene and has a few witty lines. She is the key to the whole entire plot!
Sorry, but that’s just not enough. I’ve read stories where the main character is not the main character. Sometimes, there’s a gaping hole where the main character should be. Other times, there’s a sidekick who’s practically demanding to be put in the spotlight. Either way, the story won’t work like it’s supposed to without a strong protagonist, the right protagonist. Here’s a checklist to make sure you have one—we’ll call him/her the “MC.”
Does your MC do things? I’m not asking if your MC has a lot great lines or internal struggle. Does he or she actually take initiative and respond to events instead of just being a useless pawn?
Is your MC the most developed and interesting character in the story? It’s fine for secondary characters to be quirky or mysterious or hilarious, as long as they don’t overshadow your main character. The reader needs to care more about the MC than anyone else, or the focus won’t be on the primary plot, and things will feel off-balance. (You can get away with one exception here: your antagonist is allowed to be just as developed as your MC.)
Would I want to meet your MC? This is about likeability, and does not mean your MC has to be incredibly good looking, talented, or perfect. Those are not the kind of people I want to meet and be friends with. I’d probably punch them in the face. What we’re going for here is flawed but loveable. If the reader spends most of the story hating your MC and hoping he or she dies, you’re probably doing something wrong.
Does your MC play well with others? Character interaction is incredibly interesting to people because, well, we’re people. Love stories especially tend to capture our attention, but any kind of tension with others ups a character’s interesting-quotient. Make sure your MC has people around to annoy, get irritated at, care deeply about, and have power struggles with. It’s more fun that way.