They don't always have to be lovable, or someone you would want to hang out with, but you have to believe that the character would behave exactly the way the character is behaving or the character loses credibility.
To achieve this you have to know your characters.
The first step is in knowing what your character looks like. I love istockphoto.com for this. I found this picture which I used as my inspiration for my character Ali.
I was lucky that the cover artist was able to use this same picture on the cover, therefore everyone knows just what Ali is supposed to look like.
But even if you have a picture you should know more than just the character's eye color and hair color. You need to delve a little deeper. One way that I do this is by having my character write an essay entitled "Who Am I". It's a cheesy writing assignment apparently given by every teacher my characters have.
This allows me to get into the head of my characters and find out what makes them tick, and not just in relation to the story at hand.
Lately I've been revisiting a historical fiction piece I've been working on now and then. I found pictures for all my main players and am now busy trying to find out a little more about them. Of course this is complicated by the fact that my story is set in 1775 and these characters would naturally have different concerns than teens today, or even teens back in the day when I was a teen.
I'm hoping that by getting a good grip on each of the main characters I'll get a firmer idea of exactly where the story is going and how to get it there. As each character becomes more real to me and reveals more of their hidden depth, the story also grows in depth.
It's a win-win.
What do you do to help create believable characters?