Your main character that is.
Could you answer any question posed them? Could you answer an interview of them? If you know them well enough to do that, then you’re on good ground. You know how they’ll react to most situations, even if it’s not a conscious knowledge.
I can tell you from experience, when you know your character well, they will surprise you. Things happen to them and their reactions may not be your reactions but they won’t let you make them do anything else. If you don’t know your character then you my find yourself struggling with how they interact with people, environment and situations.
My advice? Find a questionnaire, it doesn’t have to be a long one a page or two will do, and fill it out for each of your main characters, anyone who plays a decent size role in your story. Personally, I like the ones with off the wall questions like what does she wear on an average day and what would she wear to a wedding? funeral? etc. They make you actually think about your character and his/her personality. Once you’ve answered a page or two of questions about them you start to get a feel for them, and that makes writing about them worlds easier.
Do you have a favorite questionnaire? Feel free to share the links.