When I write a story, whether or not I've gotten a plot yet or not, or just the idea of the general plan, my main character usually comes first.
No matter how great an idea is, as soon as I get a storyline in my head, the first thing I think is, "Now, who will be the person that gets to have this adventure?"
I have to establish whether the material is suitable for a boy, or a girl. (For now, he's a boy. ) Once that's settled, I can start getting the MC's features sorted.
I have to decide on overall features, first. Will the MC have brown hair, or blonde? Will he be a red-head? What origin will be demonstrated in build, features, skin colour? Spanish? German? Will the MC be a tow-head of Polish descent, or olive-skinned in the manner of Italians? Will the MC be short, stocky? Tall and lithe? Small but slender? Tall but thick like a bull?
Once overall look is established, I go in for the further detail. What happens when the MC gets angry? When he gets sad? Does he have thick, expressive eyebrows or thin emotionless ones? Does his mouth get line-tight under pressure, or does he chew on his lips? Does he use his hands to express himself? Is he voluble, or quiet? What signs does he portray to signify his mood?
Once I've got that all established, I go into conflict. What moves my MC? What drives him through the story? What catapults him into the plot, to make the story activate? Why does he react this way, or that?
Once I've asked all my questions, I've got my developed MC, complete with flaws and weaknesses and strengths, ready to make my story a reality.
What's even better is that, now with Alice Orr's helpful suggestions, I can even take him one step further and write his thoughts into a journal. Don't you love becoming your character?