I have read that it's best when building a character one should
write a simple biography of the character from it's birth to grave.
Now if your writing a series you probably would want the charcter
to grow according to the story. Kind of like Harry Potter did in that
Make sure your character works well for the story. Though stories
are usually wrapped around the character it might not fit.
Try to make your character likeable for the reader. Not just a goodie
two shoes, but have something people like or respect. Perhaps even
something they would like to be. Say very smart or inventive.
Sherlock Holmes treated others terribly, but he very smart. That is why
most people liked the character, and a good mystery is always popular.
Also one doesn't need to work up biographies for all characters, just
the important ones in your story.
There are some good books on building characters. If you interested I can
mention the one I read.