Nanako (ななこ?), also known as Seven, is the spirit of the Seventh Holy Scripture, a "Conceptual Weapon" wielded by Ciel and the embodiment of a scriptural repudiation of the doctrine of transmigration of the soul. While originally only referred to as Seven, even by Ciel, the spirit of this Scripture was named Nanako by Arihiko Inui after the Scripture was thrown into a river and found by him.
The Seventh Holy Scripture was created from combining the horn of a dead Unicorn that had turned into a nature spirit and the soul of a young woman, who gave her life willingly in order to achieve this. The spirit of the human was of a much lower rank than the nature spirit and was thus absorbed in, but as a result, it acquired knowledge as a human, and gradually acquired a human's value. She saw her own funeral and her mother die of sadness, as the spirit of the unicorn, after her mother had been compensated for the death of her daughter she had felt guilty and did not survive much longer despite her ability to financially. Struck by this loss, the human spirit withdrew into the unicorn spirit. It took her a few hundred years to gain control of herself, and it took another few hundred years to gain a body. As a result, even though it took her a thousand years, she still has a large influence on her visual appearance by the unicorn, as seen by her tail and hooves.
She complains a lot about her current master and how she is treated, but she is still fairly loyal to her, as she has never had a master strong enough to make her materialize. Plus, that master was the first one to speak with her. It seems that only those who have formed a contract via blood can see and hear her at times, yet at other times others have indeed witnessed her, making this unclear. Also, some of the 'modifications' that Ciel added to the weapon appears in her design, like the dots or the ring.
Nanako appears in the Kagetsu Tohya sidestory, "Nanako-chan S.O.S."
Nanako appears in Episode 2 of Carnival Phantasm, sitting out the Tsukihime/Melty Blood-franchise beach volleyball game and about to enjoy a plate of curry in front of a beachside curry shop. Ciel, eager to make MVP in the game, chases down a volley that ricocheted off her own face but collides with Nanako at top speed. As the dust settles, Ciel ends up holding the plate of curry with the ball lying next to a dazed Nanako in the ruins of the curry shop.
Nanako appears again in the Sharing Table segment of Episode 6, sharing a table at Ahnenerbe with Archer. Both joke with each other about the failings and foibles of their respective masters, only for an irritated Neco-Arc to guide said masters to the neighboring table. The next shot only shows the damaged table, presumably the result of Tohsaka and Ciel violently objecting to their servants' mockery.
Nanako is a spirit that can only be viewed by someone with a connection to the scripture. Arihiko, after getting his blood on it, is able to see her until it is wiped off. She can interact with the world and eat food.
 Tsukihime Dokuhon PlusPeriod - TSUKIHIME Dictionary: Nanako [Weapon name • Name], p.186
Nanako [Weapon name • Name]
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".
 Melty Blood Act Cadenza PS2 Manual - Dictionary: Seventh Holy Scripture [Weapon name • Technique name], p.052
Seventh Holy Scripture [Weapon name • Technique name]
Ciel's Arc Drive.
The conceptual armament that Ciel is authorized to carry. Although it is called a scripture, it's actually an enormous pile bunker.
An iron hammer created by the Church in order to renounce the heretical doctrine of transmigration. Over a millennium has passed since it was first forged, and there's even an elemental dwelling within it, but…
Before Ciel remodeled it, it was only effective against spiritual entities. Now, however, it has been transformed into a lump of iron that can pound a vampire to death with its physical attack power alone. It has a gross weight of 60 kilograms, and a maximum weight of twice that with optional attachments.
During Ciel's Arc Drive, she drives the stake into her opponent. Afterward, what is ejected from the magazine is not an empty cartridge, but pages of the holy scripture. For vampires, this is no different from being directly injected with the holy scripture itself. There is no doubt that it is a surefire "Immortal Killer".
 Tsukihime Dokuhon PlusPeriod - TSUKIHIME Dictionary: Seventh Holy Scripture [Weapon name], p.184
Seventh Holy Scripture [Weapon name]
The conceptual armament that Ciel is authorized to carry. An iron hammer created by the Church in order to renounce the heretical doctrine of transmigration.
It was created by felling a miraculous beast said to consume souls, then using its horn as a key. The surface of the horn is inscribed with tightly-packed complaints about how "transmigration is really lame".
Since it's been almost a thousand years since it was first created, it even has an elemental living within it.
It was sanctified over the course of many years, but eventually fell into Ciel's hands.
"A horn!? That's not practical at all. The higher-ups are totally out of touch!"
Unable to hold herself back, she set about improving it, and because of that the Scripture's elemental ended up having the essence of its personality warped.
Before Ciel remodeled it, it was only effective against spiritual entities. Now, however, it has been transformed into a lump of iron that can pound a vampire to death with its physical attack power alone. It has a gross weight of 60 kilograms, and a maximum weight of twice that with optional attachments. It's a… Gatling scripture!?