In your opinion, which hero has the most charisma in the campaign?

Personally, I don't respect trickery, but the kind of trickery Biara employs is even less appealing. She's simply dishonest and doesn't hesitate to use any means necessary, which is hardly a sign of intelligence, but rather of foolishness.
Markel, while a deceiver, executed his deceptions with a certain flair. And, strictly speaking, he didn't promise anyone anything, and he fulfilled what he did promise (Isabelle wanted to resurrect Nicholas – and sure, here you go, what's so hard about it :D). Moreover, he achieved far more than Biara: she merely utilized the Lord's plan and her influence over demons, which she also acquired with his help, plus the demons' ability to shapeshift. But Markel rose from nothing, becoming that after Fiona's death, and went on to rule an entire Empire, making it so powerful that even demons with their countless legions feared it (remember the fourth campaign, when one of their generals initially refused to carry out the Lord's order). And he was defeated only by a united trio of heroes, who later, together with Railag, utterly crushed the Lord himself, which speaks volumes about their strength.