Nanako

The elemental residing in the Seventh Holy Scripture. Although Ciel calls her Seven, she was named Nanako by her temporary master of just a few days, Arihiko.
A young girl with a mysterious character. It's hard to say if she's noble or nasty. She can't help but lament the tyrannical treatment she receives from Ciel, but if she ever fired back she'd just get in trouble. Basically, she's a character at the very bottom of society.
Originally she was the girl offered up as a sacrifice during the creation of the Seventh Holy Scripture, but after her death she merged with the holy beast used as the scripture's foundation and ended up with her current form. Her most distinctive characteristics are her blue, sci-fi bodysuit and her hoof-ish hands and feet.
Though she is the guardian spirit of the Seventh Holy Scripture, that doesn't mean that she's the guardian spirit of the Seventh Holy Scripture's wielder as well. The reason that she complies with Ciel's orders regardless is because Ciel is the first master she's had capable of giving her form. Manifesting a guardian spirit isn't something that can be accomplished without an excessive amount of magical energy, after all.
She's always grumbling about Ciel's violent treatment (of the Seventh Holy Scripture), but nevertheless she sticks to her like glue. In Melty Blood, she appears briefly during Ciel's stage entrance and Arc Drive.
By the way, in the third character popularity poll Nanako's Arc Drive was listed as “Holy Cross Burial Scripture,” but it should be read as “anti-reincarnation.”

Tsukihime Dokuhon Plus Period: Tsukihime Dictionary Revised