This chapter is probably going to have multiple parts.
What I do first, and what I recommend you do first, is find what personality you prefer for them. You can do this by thinking of character traits you like for them, or if you want to go more in-depth you should probably check out psychology websites like 16 Personalities and look through those.
It is important for your character to have an equal amount of pros and cons. (Also: if in a fantasy setting, you do not want your character to be too over-powered.) it is important to avoid perfect characters, because it makes them less relatable and less human. Which takes away from the fun of making an OC in the first place.
