Another thing to think about is who your character’s family is. I know I would not be who I am today if not for my family. Whether it’s the quirky sense of humor or the fact that as the youngest I’m kind of a “mommy’s girl”, my family is my support network. I know I would be much different if I had been raised in a faster paced home or a dysfunctional one or one where my parents worked 24/7 and never had time for us growing up. I’d be much different if I was in the foster system, always getting shuffled between families with no dependable link or I would cling to any link, such as a sibling, as the only “solid” thing in my life. Also, people with no siblings or only one parent are raised differently than people with both parents raising multiple kids. Adopted kids often have things to work through such the question “Why was I given up” that they need answered or have to learn to have peace with not knowing. I have a friend who was adopted in grade school and still has contact with all her siblings overseas. That can add a whole new list of worries as can remarriages and even in-laws. I’ve been lucky with my in-laws so far but what if my brother marries an attention grabber? That would change the family dynamics at the Christmas party as my family and in-laws are rather laid back. How has your family affected how you are today? I’m sure you can think of a number of things whether it’s religion, morals, level of emotional honesty, or just a sense of humor.
Another thing that affects us at a young age is our friends. I know throughout high school I was mostly friends with the drama kids or the artsy ones. Now I have trouble fitting in with the girls I work with, the ones fascinated by fashion trends and being popular but seem to have no other hobby than hanging out at malls with friends or flirting with boys, and I have little to talk to with the boys who prefer to play video games and flirt with girls or go drink all weekend then pickup a book or make something with their hands. Who my friends were growing up made me comfortable to be me. Most people have friends or acquaintances like that and they may pop up in the story, whether to annoy a character or explain something. Also, friends can affect a person’s social confidence. A girl or boy who had few friends growing up could be really shy whereas a popular child may be very outgoing and possibly overconfident as an adult.
Another thing to remember is that how someone interacts with friends can be judged. A love interest may be watching how a guy reacts to girls in his circle or how a girl reacts to her coworkers. That could be an interesting scene if played right. No matter how you decide to use your character’s friends, or lack of, it will affect the character in some way throughout their life so you may want to at least decide how popular in what crowds they were and how that affected them, even if only for the duration of the story.