Reinforce I

Reinforce is a key character in Magical Girl Lyrical Nanoha A's, who reveals herself near the end. She does not appear in either Magical Girl Lyrical Nanoha or the alternate universe shown in the Triangle Heart series. She is voiced by Sanae Kobayashi. The former Book of Darkness and creator of the Wolkenritter (Signum, Vita, Shamal and Zafira), Reinforce was reformed by Hayate Yagami and became her partner. Reinforce is a Lost Logia and an intelligent creation of Belkan mages whose names have been lost to time. In Magical Girl Lyrical Nanoha StrikerS, she is reformatted into the much smaller Reinforce II.

Hayate, inside a void in the Book's separate dimension, opens her eyes, but the Book tells her to go back to sleep and escape the tragic reality. She could stay with her friends forever, but it would not be real... Hayate cries out that she did not want this at all. Absorbing the knowledge of the Book of Darkness, she realizes the truth of everything that has been going on behind her back, the innocence of Nanoha and Fate and the sadness of the Wolkenritter. That cannot be forgotten, Hayate asserts. She is the mistress, and the Book has to do what she says.

Just as the Book has Nanoha cornered, Hayate takes control of her body again so that Nanoha can disable it and Hayate gives the Book a new name, "Reinforce". (Source: Wikipedia)