She's not the last boss LOL. She's supposed to stay there and watch over the ends of the world for her entire life as a "Period". It's the place where the story ends and then starts anew so there time and space can be finicky and sometimes irregulars like Last Boss characters pop-up without following the proper procedures of the story. Her job is to deal with those. However, the sly fox is bored and doesn't want that (she wants to live all happy go lucky all over the world traveling and having fun). She found MC who has the potential to be a great "Period" so she lied to MC and forced her duties upon him. The reason why he's made the Innkeeper is so that he doesn't think of pursuing his own adventurer dreams and neglect killing the last bosses that pop up there.