Luc can't summon monsters from the World of Emptiness or World of Endless Night (where monsters are believed to come from and also one of the Million Worlds). He summons a wind golem using a spell taught to him by Leknaat in Suikoden 1 which utilizes his mastery over wind runic spells, but that's probably the extent of his summoning ability. He uses Sarah in Suikoden 3 to do all of his demon summoning.
He is a demon, that should be clear at least. That he is not human has been known for a long time.
Well, it may be "clear" that Yuber is not "human", but we don't actually know what Yuber truly is. We don't no for sure if Yuber is a demon or not. Yuber's actions are believed to be not human in nature and Yuber obviously distances from the frailty of humans, but Yuber could just be making claims as Yuber does not want to be associated with such a weak race especially after gaining so much power.
People see Yuber as a demon because of Yuber's inhuman love for chaos. Even after all of that, I will state that I agree Yuber is not "human", but I just wanted to state that there is no way we can know for sure that Yuber is a demon and not human as of yet. Sorry for being nit-picky on the Luc and Yuber thing.
Yuber isn't anything that big. He just wants chaos and seeks it all the time. He is even more simple than a human being. A one track mind with a simple purpose. I don't even think he appears to be very intelligent at all. Just monstrous and bloodthirsty
Well, you may believe Yuber isn't anything big, but the creators clearly state that the conflict between Yuber and Pesmerga can only be concluded in the final Suikoden. This sort of shows that Yuber is a big deal in the Suikoden World as Yuber's potential demise signals finallity to the Suikoden story.
Also, Yuber may seem to have a one track mind, but lets see what Yuber is trying to do. Yuber seems to want to cause chaos. Chaos is an effect that changes the norm of the world. The norm is order. If things stay the normal status quo then there can be no change.
Yuber is in actuality effecting change on the world of Suikoden by causing chaos. Yuber is allowing for growth. Yuber is trying to break the chains that bind mortals and other creatures to the will of the True Runes. Chaos isn't so bad when you look at it that way.
Sure, Yuber can be seen as a lunatic, driven by murder, but it is possible that Yuber's actions are not just simple-minded and Yuber's actions are of a deeper purpose. Yuber's actions could be for the good of the inhabitants of the Suikoverse.
Just look at the people who summon Yuber to work for them, they clearly were not absent-minded people. They saw Yuber as a necessary tool for their goals. Yuber is more than a blood thirsty killer in my humble opinion. Like I have stated, Pesmerga's motives worry me more than Yuber's.