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, demi-humans, demons and us readers alike while Sieg does all or almost all the actual hero work off-screen. Sieg is probably for the most time disregarded by denizens of this another world because:
a) hero is (almost) always human
b) people usually pay no heed to ants and what they are doing, even awesome ones like Sieg.
I agree with people saying this manga is probably soon ending, unfortunately. I do hope Akatsuki Natsume-sensei will shed some light on unanswered questions, but I'm not sure he will seeing how he left some loose ends in KonoSuba (for spoiler reasons I wont go into details).