This post is going to work off the premise that your book has a single main character. If you haven’t read my post on why you need to have a single main character, you can do that here. If you’re already on board with that idea, carry on.
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?
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.
Then you just have to worry about keeping them likable. But that's an easy test. Beta readers and writing groups can easily tell you if your character is coming across is likable or as a jerk. As a general rule, the meaner the character is, the more competent they must be. People love Sherlock and Loki, who are absolutely terrible to other people (both have killed people) but are also brilliant and clever and one step ahead of everyone. On the other hand, people also like Anne of Green Gables, who pretty much screws up with everything she does, but is nice and sweet and genuine to the people around her and trying very hard to be good.
Maxing out either of these traits in the same direction is going to be a problem. If you have a mean character who is too incompetent or a nice character who is perfect at everything, those are going to cause you issues.
If your character is somewhere in the middle on both, then it's time to think about making them relatable. While it can be funny to watch a millionaire whine about sitting in business class, it's not going to be relatable to most people. It's not going to get them on your character's side. But all people have something in common with other people. So you (probably) don't need to change your main character, you just have to change the way you introduce them. Make your first couple of scenes a way to show readers that your main character has interesting skills, even if they aren't the world champion at anything. Make the first couple of chapters a place where your main character is going through something that your readers will sympathize with. And make your first couple of chapters a time where readers see the character around the people they care about, where they see them being a good friend, mother, father, brother, sister, pet-owner, neighbor, coworker, etc.
If you're having trouble figuring out how you've done at creating a main character, here is a fun test that will help you see if you've reached true character creation or if you're creating an alter ego or perfected being to act out your plots. The list goes over a lot of common red flags that might scare people off from your character before they've gotten a chance to know them. A few things from this list isn't bad. But if you start to hit an awful lot of them, it might be time to add some more realism to your character.
The other important factor in creating a main character is going to be making them the right age and background to relate to your readers. Depending on your genre, 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). Mystery readers want to have characters who are detectives and CSI people and someone who is going to be clever and interested in the case. Fantasy readers want a main character who uses magic, whether they learn that ability throughout the book or have to possess an artifact. 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.
If you have a character who suits your genre, matches the age of your readers, is active in the plot, and balances out their skill level and helpfulness, then you are off to a great start at connecting to your readers and making them care about your book.