I agree with Galastel; to the point I almost did not answer!
What I can add is that my main characters, perhaps like Harry Potter, have something special about them, a rare talent or natural ability. They are crazy good at something, which may be anything from card tricks to seduction, math to athletics, whatever.
For me, the talent comes first. Harry Potter is "The Boy Who Lived," marked as special for all to see by his lightning bolt scar and his special, natural, magical ability.
But I don't start writing my character until I imagine how her rare talent would have influenced her life. With that ingredient, where was the drama and trauma, growing up? How was it trained and nurtured, if it was at all? Who knows about it? What enemies resent it or hate it or covet it, and want to possess and control it, and therefore possess and control her?
How did she discover it? When was the first time she used it? When was the first serious time she used it; i.e. not for play or some fun, but to save herself, or make money, or punish somebody, or make a real difference in some outcome?
That rare talent will define much of the character, and present me (the author) with some arbitrary choices to make, the milestones of life that may or may not depend on the talent. I must decide upon an inciting incident that triggers the story, thus her age and relationships at that time.
As Galastel says, your decisions must be consistent, but I don't bother with cheat sheets to plan the character, I want her to grow up with this talent defining her life.
Being consistent, and showing the reader how the character makes their decisions, provides enough clues for readers to fill in the blanks. If my hero takes a lover, it will make sense with all the rest of her life growing up, and the reader will see that, and know if this is a lover that makes sense for our character. They get in her head and know, not just the specifics of what she wants in a lover, but the type of person she would even consider for a lover. The same goes for her other choices and actions in life, we give the reader a template they can use to reason about our character and her life and decisions. How and why she acts, gets angry, gets happy, and navigates her way through whatever conflicts or trials we put her through. From an authorial point of view, those are there and chosen to define some aspects of her.
We need to use the specific concrete choices we write about as clues to her inner character and belief system, so the reader can predict what she will do, and knows her well enough to predict in different situations what she will do or why she would have done what she did.
Think of it as a predictive model. All you truly have, for real people in real life, are a finite number of experiences with them, of what they have done and said. Even your lifelong friends. From that finite list, you have constructed a fairly accurate predictive mental model of them, what they will say and do in various situations, even if you have never seen them in such situations.
In fiction, we have an advantage over real life: I can show you the actual thoughts and actual feelings of my characters. Those are something you can only intuit from another person. This gives me a way to shortcut you into truly knowing them, a way to build that mental predictive model faster, so you feel like you know them as well as a real person: As long as I don't do anything so incongruent with your previous perceptions of their character that your immersion in the story gets shattered.