Folken Fanel is Van's brother who disappeared 10 years ago when he went
to slay the dragon as part of the rite to become king. As he faced off
with the dragon, he lost his right arm. The 
Zaibach
took him in and gave him a new one, a mechanical one. From that point on,
Folken Fanel was no longer. He became Folken Strategos, part of the evil
Zaibach empire.
Folken revealed who he was to Van in episode 4, and at first Van was amazed
but then became angry because Folken had turned to the Dark Side. Folken
asked Van to join him in the Zaibach empire, but his brother refused, so
they are in this war against each other. Although, they don't seem to want
to battle each other, they never stop trying to
get the other on his side.
Folken, like Van, has wings and is able to fly. But while Van's wings are
white, Folken's are 
black,
which suits him: a Dark angel who wasn't bad to begin with.
Folken works with Dilandau, and usually is the boss around the fortress.
He takes orders from Dornkirk, a very old guy with a fate-changing machine.
Van isn't the only one with a catgirl who's obsessed with him. In fact,
Folken tops his brother: He has twin catgirls who adore him and obey his
every command, Naria and Eriya (aka Silver and Gold).
Although Folken may seem like a bad-to-the-bone kinda guy, he really isn't.
He's just been mislead, as you'll learn much much much later in the series.
As the show continues, there will be much more told about Folken, and the
truth behind his disappearance. And for those of you who love his wings,
don't worry, he'll be showing them (for real, not just a shadow) later
on in the series. ;)