So the protagonist is the best find of a great hero, and is a part of his party alongside the rest of the cast.
Opens up like the end of a typical battle anime adventure, but he protagonist discovers a dark secret about the hero: He's been working behind the scenes to make sure he has villains to fight against so that he could never stop heroing and his adventures would never end.
You see, before the start of his first adventure he had nothing going for him, no place in the world, becoming a hero meant living a far better life than the one he had before.
It was on this first adventure that he met the protag and his love interest, it was then when he became someone important and lived with purpose. It was then when he had the novelty of adventure to cut the banality of living in peace.
So when the first adventure ended in victory, and he saw an opportunity for another journey, he intentionally allowed a second villain to rampage just so he could have an evil to vanquish.
Even during this second outing, he was secretly setting things up for the trilogy. Since then most of the enemies they fight are the result of the hero's machinations.
The villains themselves are the real deal, they are actually carrying out their plans and causing a great deal of harm, innocents die before the hero and his allies stop them.
To be fair they do what they can to stop the disasters they cause, but now that the protag knows his best bud is the true culprit behind this and many other threats, all because he's addicted to adventuring and being the hero.
He's also much more powerful than he's revealed, holding back purely for the sake of his rivalry with the protagonist, and he's well aware of the love interest's affections for him, but never acts on it even though he reciprocates her feelings simply because he wants to keep the chase going.
So the protagonist does what he can to expose him, and this becomes a cat and mouse mind game between him and the hero which is going on during their adventure.