Chrno Crusade

Chrono Crusade is set in the height of the Roaring Twenties, where jazz is king, bootleg liquor flows freely, and the mob rules the streets. It is a time of prosperity, luxury and decadence, and the division between rich and poor grows even wider in the wake of the First World War. It is at such times of great change and upheaval that the dark things that lurk below the world of man can come to the surface. In the world of Chrono Crusade, a fictional organization known as the Order of Magdalene (or more specifically, the characters of Sister Rosette Christopher and her soul-bound demon partner, Chrono) exists to fight the demonic threats that appear with increasing regularity across America. Both Rosette and Chrono are revealed during the course of the story to be driven by a shadowy past, centered around a search for Rosette's lost brother, Joshua. Joshua is shown to have been taken from her by the sinner, Aion, a devil who shares a dark and bloody history with Chrono, and seeks nothing less than to overthrow Heaven and Earth. (Taken from Wikipedia.)

Chrno & Rosette

By a twist of fate, Rosette and Chrno end up being partners, although humans and demons are supposed to be enemies. While searching for Rosette's brother Joshua, they come across various hardships and through this learn to trust and care for each other, to the point that they would trust each other with their lives.