Nosteo is clearly of Chelish decent, he has the pale skin, dark hair, and sharp features associated with them. Nosteo is from Varisia however. In his youth it became clear that he carried the family curse, which only shows up every few generations. Somewhere in his family's past a devil's bloodline was added into the family tree. No one remembers for sure how that happened but the family also passes down an ancestral longsword that seems to be a Hellknight weapon and it is commonly accepted that it has something to do with the knights. It's also probably the reason why his family now lives in Varisia.
To try to combat the evil blood in his veins Nosteo joined the church of Iomedae. He tries his best to be as honorable and good as possible but he has found he is a very good liar and slips up from time to time.
Nosteo found out about the origin of his curse-the little known about it anyway-after his father bequeathed the family sword to him. He decided to find out everything he could about the Hellknights, and maybe his family's history, that day. When he heard about Citadel Altaerein he decided to go check it out.