This is what drives your plot. Without a problem, you don't have a story. You don't have to write a biography of your main characters for your readers. Naturally, in a novel, you have more room for description. However, you still cannot get carried away with it and bore the reader.
Nobody is perfect.
Your hero or heroine won't seem real if they are faultless. You can endow them with a minor negative trait to make them appear more human. Use active verbs. Watch out for an overabundance of WAS and WERE in your story. Stay focused on ONE theme.