My theory is Sieg is actually the hero being summoned while Genzou somehow got caught in it due to some freak magical accident/coincidence. Or maybe summoning magic summoned both of them from their respective worlds as co-heroes where Genzou (inadvertently) attracts all attention from humans...
Damn, I haven't read that manga yet but I might give it a shot.
I came to your page because I was almost certain it is Excelia from Nukenai Seiken-chan.