Not necessarily - but you should write a Character Sketch. You need to at least know your main character's likes & dislikes as well as a favorite item or two, favorite color, food, and other such things - and don't forget MOTIVE! Can't have a story without a MOTIVE!!
There's a balance - I'm writing a manga right now, and I did get some information about my lead male and lead female. However, what makes a story realistic is when the characters tell you about themselves and they start telling you what to write.
It's important to build a foundation, but once you do that you have to make sure you're characters have room to grow, and if you want it to be believable they have to grow on their own.
I believe that you should know something about your characters before you begin. Things like their age, appearance, and personality. I recently wrote a short story in which all I had for my characters were names. I had to go back and revise so much due to miscalculations in age and time span as well as inconsistencies regarding personality. That experience taught me that it is best to know your character before you introduce them to others. Imagine introducing someone to your fellow friends as someone important to you after realizing that you know nothing about them! lol. Our characters are our inspiration. They are the ones that we are writing about and knowing them is as important and knowing the kind of story you want to write. Hope this helps