The Princess of the Tower wants a Hero

Princess Lumi has been confined to the demon lord's castle for long enough to have strong opinions about it. The castle is in a land of everlasting winter. It is cold. It is lonely. The food is not good. There is nothing to do. The only redeeming features are the tea and snacks. A hero arrives. Terms are reached. The days of winter they spend together are what the story is about.