Zatch Bell!

At Kiyo's door, a man named Renji shows up and reveals that the dog, Gofure, is his Mamodo. When he casts Doruk, Gofure transforms from a small dog to a ferocious demon. Kiyo hears Zatch's scream for help and rushes downstairs to help him. Zatch has lost his will to fight due to Kiyo's harsh words, and is severely beaten by Gofure. Brago and Sherry step into the fight and quickly defeats Gofure. Sherry takes Renji's book when he runs away. Brago and Sherry then turn to Kiyo, who is laying on the floor, barely conscious. Sherry casts Gravirei to crush Kiyo with gravity, and warns him he will die if he doesn't hand Zatch's book over. Despite the pressure, Kiyo refuses to give up the book and resists longer then other humans normally would. Sherry then casts Gravirei on Zatch, but Kiyo still refuses to give the book. Kiyo then begins to walk towards Sherry, causing her to cast Reis on him. Sherry, upset that Zatch and Kiyo are still standing, casts Gigano Reis, a bigger form of Reis. Kiyo casts Zaker to cancel both of their spells. Kiyo then walks up to Sherry and tells her they'll never give up the book before passing out. Sherry grabs Zatch's book, but realizes the bond that Zatch and Kiyo have, puts the book back, and warns Zatch they will return. While leaving, Brago notices that Sherry looks unusually happy. (Source: Zatch Bell Wiki)