For me, I actually not 100% trust him (but I choose to trust him in gameplay btw). According to Vicoria, she said that 'Feliar' isn't his real name and Nameless tell us about 'shapeshifter' friend we have. So I assume that Feliar is the one who cause Orc's War and Nortcrest Conspiracy. Another reason, his appearance make me think about 'Satan' somehow.
But it just my opinion. He may turn out to the most trustful person in the next period of story, I don't know either. But for sure if you choose to not trust him in gameplay, he will like you less lol