Every book is driven by at least one main character, a singular hero
or heroine who propels the plot. Often, the main character has to face a
challenge or foe, obstacles they must overcome in order to get to the
happy (or tragic) ending. But if that character doesn't reach out and
grab your audience, readers may not stick with your story all the way to
its end. The main character is the driving force of your book, so make
Who is this person? What motivates them? What are their hopes and dreams? If you don't know, your readers don't know. Readers want a character they can identify with, a three-dimensional character who has their own independent thoughts, wishes and yes, even problems. Identify and know the characteristics of your main characters, who they are as a person. Nervous habits, likes and dislikes, opinions -- these are the things that make a person real, and the same stuff makes characters real to readers.
No one is perfect, not even in books. If you want to craft a main character that others can believe in, you've got to give them flaws -- or at least one major flaw. Readers are all real people, and like real people they make mistakes and bad decisions. Your character should have some flaws, too, and shouldn't always do everything right. Some of the most lovable literary characters are very flawed, but people embrace them because they can relate.
What does your character look like? Tell your audience in detail what they might see if they look at your character. You want your main character to jump off the page, and that means giving them a physical appearance so readers can "see" them within their own minds. A character that your readers can envision is a real, relatable character that will jump off the page.
Physical flaws are important, too. Ask one hundred people if they dislike something about their appearance, and one hundred people will say yes. What doesn't your character like about his/her appearance? Perhaps they have pale skin, a cleft chin they hate, wide shoulders or 15 extra pounds hanging around their hips. Why would a reader, who has all the normal hang-ups that come with being human, read a story about a physically perfect human being? That's not relatable, and it's not very realistic unless there's a good reason for it (they're a vampire, they've had plastic surgery, etc.).
The key to writing a believable, relatable main character is in having a clear idea of who that character is. Make sure you know the person you're writing about, even if they're someone you've made up in your head. If the character feels real to you, it's more likely to feel real to your audience. A great main character can make any plot shine and stand out above all those other books out there.