He is wary of everyone because of Galbatorix' meddling.
He is as brilliant as a ruby and shines very brightly. He is a bright, bloodred color. He is a very large dragon. He should be slightly smaller than he is, but due to Galbatorix' meddling in Thorn's youth, Thorn is bigger.
Murtagh is his rider, and he is very good with a sword and very powerful. Murtagh wields Zar'roc, or Misery.