It may not seem like this at the start, but you know your central character very well. Creating them in writing is a matter of trusting your knowledge, of discovering the person rather than making them up. For many writers it is the most instinctive part of the process. The slightest sketch of a plot will contain within it the germ of the protagonist, who will become a fully rounded human being as the story develops. Like a new-born baby, that germ is perfect in every detail and all you have to do is nurture it.
Many books start with a character.
What anchors the writer's attention may be a few words of overheard conversation, the way a friend keeps smiling as she screws her life up, the look in the eyes of a face in a photograph. Occasionally an entire person will simply walk out of your imagination, so fully realised that you know them as well as your oldest friend and never hesitate in deciding what they will do or say. More commonly, only one quality of the protagonist's character will be clear at the outset. The creature will be more graceful if you allow it to evolve from your imagination. Some writers go in for outright bodysnatching, putting a whole known person in their story.