What I suggest for you Ivanhoe is to decided what kind of person he is first, what environment he lives in, and because of that environment what he turns out to be like.
Let's start with what you have down first for a character concept. And please, when you reply, reply to each point so I know you've read what I wrote.
1. He is an orphan on Earth so he's lived with danger, the meteorites alway striking, the poverty. Its a tough life. He's basically one of those dirty street kids that are always harrassing people for money, pickpocketing them, etc. Most street kids form huge gangs made up of just kids of which they will take down adult targets and steal from them.
2. Why did he decide to leave Earth at 12 and go to Mars? Was it that he heard that Mars was basically like Disney Land? Sort of a paradise?
3. You said his mother died at birth, then who took care of him? A baby can't survive on its own. And why did the person who took care of him leave him alone?
4. He just up and decided to become a criminal? Ok so he wants money, lots of it and he wants the easiest path to it. Since he has no education he decides to join the local gangs. No biggie, it'll be like the kid gangs on Earth except now you're in the adult gangs.
5. Ok the gang taught him guns. How did he survive in the gang without having to shoot someone innocent in their robberies? I can't see him surviving for long, gaining any respect, or any trust (thus not getting much loot) if he doesn't prove himself to the gang.
6. I do like Daiymo's idea of him spending prison time, perhaps the gang foisted off the whole blame on him,double crossed him, left him to take the full blame. Now that he's out and served his time, he wants revenge so he joins the BeBop to do so.
7. Also I know this doesn't have anything to do with it, but I'm curious. I saw that you know Corlon and I know that Corlon is 13. Are you 13 also?