Priest turns out to be a relatively powerful demonkin waifu, MC turns out to be Guts (Berserk) without the rape trauma. The priest is useless in the first arc because of a magic-nullifying amulet, which later conveniently disappears because it would break in-universe balance. Afterwards, the priest tricks MC into being her debt slave, which the MC does not even realise because he's dumb as a rock. You would think that a famous veteran mercenary had a personality, but he might as well be a pet dog with a sword and the story wouldn't change.