Today, I've got Sharon's character on the couch. He's 15, of some Oriental or Asian descent, living in America. His sister brought them to America when their parents died, but then she died, and Jun ended up in the foster care system. He's an extremely angry, intelligent young man, and Sharon is wanting some general commentary on the intake form she filled out.
My initial thoughts on Jun is that it's going to be difficult--not impossible--to sustain a novel with him as the main character. He's angry, yes, and with how you wrote his voice, it comes across to the reader in an overt manner that this is one ticked off guy. It might become annoying to a reader to be bombarded with his anger over and over (at least, it was this way in the intake form).
My question is why? Yes, he's had a hard life, and suffered a lot of loss. This would make anyone mad at the world, mad at God, mad at themselves. But you want to be careful that your 15-year-old character doesn't come across sounding like a sulking 8-year-old. He's 15, and his coping mechanisms will be different, and a little more covert. On paper, he sounded like he was on the brink of a tantrum.
You wanted to know if he sounded authentic, and I think there is still some work to be done to make him realistic. Good news is that there are lots of resources out there! I'd encourage you to read this post I did several years ago about anger as a secondary emotion. It might help with solidifying the real emotion behind his front of anger, and that will aid in making him multidimensional. And then you can check out The Bookshelf Muse for some additional ways to exhibit his anger, such as working his jaw and flaring his nostrils.
Anger management therapeutic techniques are fairly easy to come by on the internet, as well. There is a free downloadable .pdf workbook and manual from the Substance Abuse and Mental Health Association. This is an awesome site with tons of free materials that are really helpful. So check that link out for additional insight into anger.
Wishing you the best with Jun!