Characters, it's what makes your book readable. You have your protagonist and your antagonist. There's your supporting characters and the characters that add a little bit of color here and there. But, what really makes your book *pop*? What really makes your characters relatable? It's their flaws.
That's right. It's their flaws. It's the bits and pieces that make them seem human, a bit more like the reader themselves. I had a friend not too long ago ask me to go over a story idea of his and give him some feedback. It wasn't too bad, but I suggested that he give the three main characters some flaws so that the readers would care more about them, and, therefore, more about the story. His reply back to me was that the one character was arrogant and that was his flaw. Maybe, but I don't see how that would make the readers want to keep reading and get invested for that character. He's since revisited those characters.
Personally, I find it easier to put something of myself in each of the characters that I write for. Maybe it's more helpful that I have a co-author and the way that the two of us play off of each other we never know what the other is going to do so it keeps things more interesting.
The bottom line is this... keep things interesting. No one wants to read about a perfect character that never does anything wrong. Give them something that makes them relatable and it will keep your reader's interest and you won't regret it.