I'd say anybody who simply got killed by Flamie would have been no good for the mission anyway. She's tough, but not that tough, and not nearly evil enough.
Like fireheart said, simply faking to be a brave would amount to nothing since entering the demon land kills common people. So, there has to be something more to it, unless the fake brave's goal was already reached. So, it's either a fiend in disguise (maybe a fiend that's wearing a skin suit of a brave, like the cockroach alien in MIB), or a person who made a deal with the demon king. I bet the demon king could grant somebody a copy of a tattoo and the ability to stand the miasma. In fact when Adlet got his mark, a thought visited me that it wasn't genuine but the demon king chose him for some reason. He's easy to fool, as we have seen, so he might be a tool without knowing it himself. The process kind of looked too pretty and thus suspicious.