Nanako

The spirit of Ciel's weapon, the Seventh Holy Scripture; referred to as "Seven" by Ciel, but nicknamed "Nanako" by Arihiko. She has hoofed feet and a horse tail. She also has the ability to transform into a winged horse, and is quite scared of her master, Ciel.

Spoiler

She is a combination of the spirit of a girl from a poor family who willingly offered her life as a sacrifice to be combined with the spirit of a unicorn's horn.