Fate/stay night, Yumekui Merry Recommendations
Both series feature "master-servant" relationships where an ordinary human being is paired with a character who possess superhuman abilities. The pair is bounded, and the death of either side will greatly affect the other.
Battles occur behind the scenes, yet the the consequences are seen in the real world.
There are mystery elements in both cases, and character backgrounds are slowly unraveled through battles.
Both have main characters that meet a magical female character who fight others of their kind.