We dont know much about Hiller's past, but he's not human anymore, even if he act like a human : this is a demon in orders of King Inma. Since he try to awake him, he infiltrate human society as a priest. He found Semalya and save her from a suicide, since, he raised her for help him to awake King Inma. He also raised Lilina but we don't know if he had some project for her before she found the crown. When he saw her wearing the crown, he acted as he would hep her to remove it. In fact, he gathered informations to make Lilina collect all relics necessary for the King Inma's awakening ritual. Since King Inma's is not awakened yet in Cursed Armor II, it seems that he failed to convice Lilina and died or Lilina never go to the Relic's room with him.
As King Inma's peon, Hiller is loyal to him. All his actions have for objective to help his Master's resurrection. But even if he try to do this, he kept some human's feelings since he want act like a real father for Semalya and Lilina. He's persuaded that after Inma's awakening, he could live as a family with Lilina.
As Cursed Armor's "final boss", Hiller is a powerful foe (need more informations)
Semalya : Hiller raised the nun like if she was her daughter. He teached her some sacred fighting skills in order to defeat demons and any devil foe who can attack her or be an obstacle for Inma's ressurection.
Lilina : Even if Hiller tricked her about the Relic's Quest, Lilina is a real family member for him : he really loved her as a father love his daughter. Even if she could not be the same after she married to the demon lord, he's persuaded that they still act as a family.
Seaclya : No matter if the are together in Inma's orders, the two demons can't act as a team. Since Seaclya's loyalty is not total, she wont obey to HIller's orders and try to interrupt the ritual as long as she can manipulate or convince Lilina to avoid it.In normal end A, Seaclya will take control of Lilina's body and kill him.
King Inma : Hiller show a total loyalty to him, but his human's feeling wont please to the Demon King. Is bad end B is reached, Inma say that he gonna wipe his memories to supress this feelings.