Pick a role. Stick to that role. Don't fall for the snare of making a master of all.
When I make an OC, I start with their philosophy.
From their worldview, I tailor a backstory that shows how they reached their outlook.
After that, I ask myself, Given their role and philosophy, what (If any) combat abilities should they have? I advise always keeping it small, starting out, and fleshing it through play.
Once I have who they are and why (Philosophy and backstory), I pen the personality pros and cons based on that skeleton.
Everything else follows in whatever order.
The last and least important element is the picture. People seldom are what they seem on the exterior. And building one that is a perfect fit for your reference means you're building a book that can be assessed by its cover. Not my preferred way of doing things.
But this is only important if you want to develop them via play. A drop-in and out character doesn't matter. You'll likely gut them and reassemble them to fit in any narrative you wish. Changing their backstory, culture, abilities or etc., will make them a new person each time. So they are, at that point, a malleable template (at best.)
If that's your style of play, then I wouldn't bother with any of this. Given it will be depended on your partner and their prompt.