Like other people already said, there are two traitors in the hero party, and a third one that's like Ageha's puppet.
About the hero title thing, Ageha is a self proclaimed Hero. She calls herself that because she really killed the Demon King, but no one knows. The Hero everyone knows is this guy and his party.
What happens is, they enter Katarov and find the Demon King. They fight and kill it. Soon after, the traitors use a artifact to resurrect him and then kill the Hero. Kiska resets in order to try and save the Hero.
New attempt, Kiska inform the Hero of the traitors, but the Hero does not believe him. Same thing happens.
Now, Kiska try to find ways to defeat the traitors. I don't really remember everything that happens, but, in the end, Kiska asks for Ageha's help, since she is strong. She helps, they defeat the DK, but then Ageha orders her puppet (white haired guy) to kill the Hero and the rest of the party. That's the reason the people of the world hate white haired humans, because it was one of them that killed the Hero in the original timeline (before Kiska shows up). One of the members is able to escape, and tell the story about how a white haired human betrayed the Hero party and killed the Hero.
After both the Hero and the DK are dead, a new entity shows up. It's stronger then Ageha and have the power to destroy the world. It says that Ageha and Kiska messed with the natural order of things, and threatens to destroy the world, unless they make a deal: seal both Ageha and Kiska in the same type of crystal Ageha was. Ageha freaks out because she doesn't want to return, but Kiska tells her this time will be different, because they will be together, and she accepts. Entity tricks them and seals both in separate crystals. Kiska experience the dispair of isolation in a completely empty space. After "serving" his time, he breaks free and frees Ageha. Arc ends and new one begins.