Having the right main character can make or break your story. The main character can determine which audience this book is going to appeal to: male or female, middle grade or young adult. They also change the voice of your story and the tone. Are they funny and light-hearted or do they take things seriously? Do they sympathize with the villain or see them as evil incarnate?
So how do you make sure you are using the right main character?
What Role Do They Play in the Story?
I could go into a long-winded speech about being relatable and empathetic and all that jazz, but really it comes down to plot. Your main character needs to carry the story all the way from the start of the fight until the dramatic turning point in the climax.
Of the two of these, the end of the book is much more important. It’s not that hard to pick a different place to begin your book. Lots of published authors do this several times with each new book they write. But it is really hard to work your main character into a climax they aren't prepared for. If your villain is a crazy computer hacker making all their threats digitally and remotely, I’m not sure what your karate-fighting protagonist plans to do to stop them. And even if you could find them something useful to do, no one wants to read a book about fast-paced hand-to-hand combat that builds up to a final long, heartfelt conversation. If you are going to follow a character who fights their battles physically, you need to give them a way to use a physical fight to save the day.
Some people get around this problem by having a side character take up a dramatic moment and underdog their way to victory. And to an extent that is a cool move. But they absolutely cannot fill in for your main character. If your main character isn't the one who plays the crucial role in the climax, your readers will feel cheated.
Imagine reading all seven Harry Potter books only to have Neville kill Voldemort while Harry stands there uselessly. Problem solved. Happy ending. Series over.
There would be an uprising. Possibly an angry mob.
Sure, everyone loves Neville. And his moment killing the snake was legit. But Voldemort was Harry’s fight. And that was the showdown that everyone waited seven books and thousands of pages for. If J. K. Rowling had given the final fight to Neville instead, she would have been chased down with torches and pitchforks.
Having a main character who sits and watches the important stuff happen is boring. Readers want to be rooting for them to win. Now this doesn't mean that your other characters can never do anything interesting. It just means that any crucial aspect of the climax (or any important moment in the plot) has to be traced back to an action by the main character, and preferably not very far back. As long as you are doing that, you're going to have a main character who can hold up an exciting plotline.
What Readers do they Pull into the Story?
The other important factor in creating a main character is making them the right age and background to relate to your readers.
Depending on your intended audience, readers are going to relate to certain traits and ages better than others. Little kids won't want to read about anyone over 14 because teenagers are weird. 40 year old readers probably don't want angsty teenagers leading the story because they've passed that age and are never going back. (Thank goodness!) Matching the general age of your readers is important. Characters of a similar age will naturally deal with issues and themes appropriate to that age range.
The genre will also effect what characters make a good main character. Mystery readers want characters who are detectives or CSI, someone who is going to be clever and interested in the case. Fantasy readers want a main character who uses magic, whether they possess that ability naturally or have to possess an artifact.
Most likely, once you have a character who is playing an important role in the climax, you have probably already chosen a main character who is suited for the genre. A main character in a fantasy novel who can't use magic is likely not going to end up useful in the plot, same as a murder mystery novel heroine who swoons at the sight of blood or would rather be studying the life cycle of sea turtles than solve a crime. But if you have somehow managed it, now is the time to make sure that your main character is suited to the genre that you are writing in. If they aren't, you may want to choose a different character to focus your story on or choose a different genre for your character's story to take place in.
If you have a character who suits your genre, matches the age of your readers, and is active in the plot, then you are off to a great start at connecting to your readers and making them care about your book.