I don’t have too much to say about characters actually. They all differ, they are all crucial to your story, and they are all carrying something of yourself.
That’s right, I subscribe to the idea that there is a little bit of the author in every character. Yes, even the murderer, and yes, even the politician. We are all.
Characters build over time. Some are planned in advance, perhaps the reason a story unfolds in the first place, whereas others are born during the outlining phase. Main characters usually pop up in your mind before you start writing since they’re probably pushing your story forward, but support characters are sometimes invented while writing. I see no problem with this, other than that unplanned characters are often introduced with just one purpose: To move the story forward. That’s not wrong by itself of course, but even small characters need to be believable, otherwise you’ll risk breaking the reader’s spell.
A good character is believable. A bad one isn’t.