Creating a living, breathing character that readers sympathize with, love, or hate is often the key to a successful novel. Coming up with one that isn't clique or run of the mill is the hard part. How many tough P.I.'s with a penchant for drinking and fast women, or teen girls pining for the untouchable guy have you read about? Far too many I imagine. Does that mean you can't put one in your story? No, not at all. However, if you want your novel to be successful all elements of your characters cannot be clique.
Your main character, or characters, must have depth. Not to say you can't have a shallow character. Every now and then I love a shallow character but even they have to have depth. Think of why they're shallow, what made them that way? There's your depth! Adding small things like habits, nuances, likes, and dislikes will make them more relatable. If readers can't relate with your characters in some way they probably won't keep reading.
One very important thing to remember is that your characters shouldn't be too perfect. If they're gorgeous, charismatic, and completely without flaws they aren't going to be someone readers will sympathize with. In reality, no one is without flaws, our characters shouldn't be either. They don’t have to be physical flaws, they can be emotional. A flawed character is a beautiful character. If you tend to make your characters too perfect make that your mantra.
How does one come up with characters? Observation of people is one of the best ways. Take a little bit from here, a little from there, and create someone unique. I'll reveal a secret of mine here. Sometimes I'll do those personal questionaires, you know the fun ones will all the questions about your likes, dislikes, and history. It creates a great spring board for ideas. Remember to have fun with it!