I know some people like to allow their characters to develop naturally through the course of the story. Others don’t know anything about their characters until they write them down. Then there are some who plan and plan and plan their characters out.
For me, it all depends on the novel.
For my George Florence series I had personalities in mind for my characters. Then I wrote them and they turned out completely different.
For my Take Over novel I had their personalities planned and so far they’re sticking to them.
For my Diary of a Lover novel, I let the characters run the show.
So you never really know what is going to happen whether you plan or not.
However, there are definitely a few things you should know about your character before writing them:
1. First and last name 2. Gender 3. Age
4. Career and/or Education