In my opinion, its main factor is environment; especially the family environment, and then school environment because children spent most of their time at home more than at school. Family is a place where they got character education for the first time. Children will grow become a personal who has character if they could grow in an environment who has character. In this case, parents have a very important role in providing character building to their children. Here, I talk much about "children" because a person's character actually is formed since she/he was child. Family is a crucial factor for the development of the children in the future. How we educate the children now is how they will be in the future. Four things that every child must-has are good faith, attitude, behaviour, and knowledge.

I'm not sure which category this applies to, but I believe the influence of our parents may play a critical role in the formation of our characters. I was fortunate to have average, kind, moral parents and I always believed that those were the characteristics I wanted for myself.

As to me, I think the first one may dominate the majority of the formation to a human's character and the 2nd and 3rd only teach us how to control our character...errr...I mean something like temper...Actually I tend to believe there is an interation among those factors...maybe the gene decides which education a person will accepted (If he/she can make the choice by himself/herself), and in turn the education he/she accepted will contribute to his/her own character or personalities.

Richard, your example of the three brothers points MORE to genetics because all three had the same loving or rotten parents raise them! What they could have gotten was a bad draw on the gene pool. 

Personality IS DEEPLY affected by biochemistry. Many developmental and emotional illnesses are genetic as well as a person's genetic ability to create the neurotransmitters and hormones that deeply affect emotions and personality. Those genes though often need to be activated by environmental factors and can be mitigated by environmental factors. Not all genetic propensities end up getting expressed. In other words, just because you have a cancer gene doesn't mean you will necessarily get cancer. You might be able to do things to make sure that gene's activation and expression never gets triggered. 

Brook, I really love what you wrote:

When you grow up though, self-education overcomes factors of genetics and environment. 

You can educate yourself on ways to play the hand you have been dealt and outsmart the house! (The house is an expression which means the gambling establishment. It's considered hard to outsmart the house but really great players can do it).