I start off with a concept... either stemming from a costume (now what powers would go nice with these, can't have a rifle because my hook for hands can't work the trigger, can't have wings because shoulder armour will shred them the minute I fly... etc.) or a power combo (now which set of tights advertise/conceal my true power...).
BUT THEN after the first few levels, the character takes a life of its own. HE/SHE/IT decides the play style, which powers to take or skip, how to slot those powers, and even which costume pieces to add or remove. I have over 100 toons, about half of them are 50s, a few are the exact same power combos or very similar; and yet none of them have had the same levelling up journey, the same set of power picks, build, nor strategy for taking down foes.
In the end, I suppose MY personality shows through as if I was the character (like what would I do if I was a half-feline alien with dark powers and bewbs). so a little bit of role playing, a little bit of real life me, and a lot of growing pains (trying out new powers, new slotting, new hairstyles and tights) go into fleshing out a particular toon.