Okage: Shadow King has fairly standard turn-based RPG gameplay, and a fairly non-standard story. The theme is a combination of humor and weirdness, which is about what you'd expect from a game whose Japanese title ("Boku to Maō") means something like "Satan and Me".
Your main character, Ari, is part of a family of generally unhelpful people. When his sister gets cursed (and I won't spoil the plot by saying what type of curse), the family tries to resolve the curse by summoning the Shadow King, Stan. This devil Stan takes over your shadow, and from there demands that you travel the world eliminating pretenders to the Shadow King throne.