When people read your book, you want them to have a strong sense of who your main character is. So if you don't know, they're not going to get that out of your pages. And when audiences can't connect with a character, the book isn't going to do well. So today we have to ask a question: do you know your character?
Think about your main character...and now interrogate yourself. What are the identifying characteristics of this character? Don't tell me what they look like -- I'm not interested in hair or tattoos right now. Is the character brave? Honest? Sneaky? Cunning? Funny? Shy? If you can't answer, you haven't done your job as a writer. Because I'm not the only one who will ask. Your readers will, too.
For each main character in each of your books, you should be able to identify at least three personality traits that help make this person who they are. Nothing that has to do with looks. Personality traits mean are they greedy? Are they generous? Hard-working? Talkative? Selfish? Mean-spirited? Kind? Be able to name at least three characteristics like this. If you can't, you've got a problem.
Think about all the iconic characters you've read. For example, I love Scarlett O'Hara. I would describe her as selfish, ambitious and stubborn. Anne of Green Gables is talkative, friendly and intelligent. Both these characters are complex, and have many more personality traits than this. But the point is, most people who have read these books can easily identify personality traits for these characters.
You want your main characters to come across the readers the same way: with strong personalities that they can see on the page. Write scenes that show these personality traits. If your character is brave, have them do something brave (like when Ponyboy and Johnny saved the little kids in The Outsiders). If they're cowardly, have them do something cowardly (like hide from an enemy).
In other words, know what your character is like. Know the traits they have. Once you've got that, it will be much easier to transfer that to the page...and that makes it possible for readers to see the traits you created. The ultimate goal is for everyone to know who your main characters is, and it begins with you.