Once you have the basics about your main character in place, including physical characteristics, likes, dislikes, skills, shortcomings, etc., you'll want to dig a little deeper into the background to make sure that you have sufficient details about him or her to carry the character through a series of books.
I suggest that you devise several of your secondary characters first. These are the three or four or more characters who will travel through life with your main character. To have them seem authentic to your readers, you'll need to devise the full scoop on these buddies as well.
Start by recording the basic characteristics about these characters and then move on to making a list of the major interactions each character has had over time with the main character. These are past events that you will be able to draw on as a writer as you write the current story. You need to make notes of these stories in sufficient detail that you will be able to weave them into whatever book in the series where they will do the most good.
The core group of sub-characters are very important to your story. Don't make them mirror images of your main character. They call them "characters" for a reason. They add spice to your story. Your readers learn their likes and dislikes over time and they learn how they will react in certain situations. As a consequence the reader feels right at home, because they can anticipate how these core characters will react.
It's important to put as much detail as possible into this back story. Don't be too anxious to write on the novel until you know you've done a thorough job on the biography of your main character and the biographies on the sub characters. When you're writing the fourth, fifth, sixth, etc., book in your series you'll be glad for all the detail and wonderful stories you wrote into your notes for later use.