Ahh...the power of darkness is a thing of beauty, a black tapestry of chaos! A tapestry on which I shall paint an all-encompassing nocturne!
Hein was once an advisor of King Argus. He was the king's loyal advisor until the Great Earthquake hit the Floating Continent. Darkness consumed Hein and granted great power at the cost of making him power-hungry. To that end, he brainwashed Argus' men into serving him, imprisoned those he could not as well as the king, abducted the Elder Tree from the Living Woods and turned it into his fortress, and lastly sent his new soldiers to pillage the town of Tokkul.
Hein's a busy man.
Hein is quite the villain by Final Fantasy III standards. He's the final boss of the Floating Continent and is built up accordingly: The party learns what he's up to while they're traveling with Desch and his castle can be seen floating in the desert near Tokkul. The remake even adds various new scenes of the party reacting to the problems of Argus, the Living Woods and Tokkul. Still, it's only after the heroes have been blessed by the fire crystal and a man begs them to stop Hein's minions from burning down Tokkul do they finally take the fight to him.
Hein's signature ability is the Barrier Shift: His weakness defaults to fire, but with the Barrier Shift, he can change his weakness to either Ice, Lightning or Fire. The game doesn't tell what Hein's weakness changed to and his evasion is high enough that fighting physically is a toss-up. As even the game tells you, this is the point to use Scholar as their abilities can let them view Hein's weakness. The remake sweetens the pot by throwing in the ability for the Scholar to double the power of items they use in-battle.