Remme wanted to be a hero, but the providence gave him a black mage job, and Phoenix wanted to be Remme's follower, but he was choosen as a hero by the phoenix spirit, one of the strongest who only answers to the ones who don't seek power. So he had to take a leader's role against his will.
The manga didn't explained the prejudgement against black magic well, and it's really confusing now, even Remme realized as a kid that his future as a explorer is going to be rough, since their jobs are pretty much determined by the providence.
The demon castle is the only place where they can...
@Firemorfox
That's pretty much it, Remme was in the party from the beginning, how they could know if there's was an effect, if it was always displayed, now that his magic is out of the table they can realize it, even Phoenix doesn't know how strong is he, only knows that he is strong.