This is a slightly strange setup.
"We summoned you just to be a figurehead in a new year festival."
So the summoned "hero" is going to lose one year of his normal life for entertainment purpose?
I'm glad there were a couple hints that this is not all there is to it or I would have been a bit mad at those otherworlders.
As it stands, this looks like they're actually trying to spy on "our" world's technology. (Magic is all well and good, but it might be limited by their imagination or resources.)
Either that, or they need to summon a hero to strengthen the peace treaty, kind of like nuclear dissuasion. How can you be sure that some demons won't try to go to war again, unless you can prove that humans can get a hero again?
I don't know for sure, but I'm curious to learn more. This kind of story, where we discover the "ever after" of a "hero kills the demon lord" story, is getting increasingly frequent, but it's a nice concept that changes from the more common battle archetype.