Hayate in A's
A's opens with Hayate sleeping in her room on the night before her birthday as the Book of Darkness on her bookshelf activates, summoning the Wolkenritter. Though Hayate is an orphan, she gets an allowance from her father's friend and lives with the Wolkenritter, whom she tells civilians are her relatives. Though she is their mistress, she sees herself only as their friend (sometimes referring to them as her "kids"), and has a good relationship with them—especially Vita, who reprocically devoted herself for her to a fault. Hayate also befriends Suzuka Tsukimura, who is, unknown to the Wolkenritter, one of Nanoha Takamachi's best friends. Hayate asks the Wolkenritter not to fight anyone, only wanting to be a family with them and for them to get along with each other. The Wolkenritter initially comply, but after a few months, they realize that the Book of Darkness is sapping her mana to fill its pages, and that if they do not fill it with the mana of others, it will continue stealing Hayate's mana and potentially kill her. The Wolkenritter decide to keep this secret from Hayate, knowing that she would not approve fighting for her sake, and decide not to kill anyone to get it.
The Wolkenritter, especially Vita, are determined to save Hayate even if it means going against her wishes. However, what the Time-Space Administration Bureau (TSAB) knows that they do not is that the Book of Darkness, once completed, may stop sapping her Linker Core for pages, but that does not mean that it will stop hurting her or undo its damage. In fact, the master of the Book dies upon its completion. Just before said completion, Suzuka brings Nanoha and Fate to the hospital as a surprise visit, and encounter the Wolkenritter there. The warring groups recognize each other and their shared wish at once, and meet on the roof of the hospital. Nanoha and Fate try to tell the Wolkenritter about what will happen to Hayate, but their confrontation is cut short by both the interference of two masked men and the completion of the Book of Darkness with the Linker Cores of the Wolkenritter themselves.
The masked men are actually Liezee twins (in transformed state), Alia and Lotte, who trap Nanoha and Fate and pose as them as they bring Hayate up to the roof and reveal that she can not be cured if the Book is finished, pretending that Nanoha and Fate killed the Wolkenritter, finishing off Vita in front of Hayate. Hayate screams in horror for all of this to stop and is taken over by the will of the Book of Darkness, becoming the fearsome possessed woman inside of it just as the real duo escapes their prison. As Nanoha and Fate fight her to no avail, Chrono confronts Graham as to why Alia and Lotte had done this. The old man reveals that he sent them, and also that he was Hayate's benefactor, not a friend of her father's. Graham had known all along whom the mistress of the Book was, and he wanted to at least give the innocent Hayate some happiness before he was forced to seal her away in another dimension with her book. Chrono asserts that Hayate does not deserve this fate and that there is another way.
Nanoha and Fate attempt to attack Hayate as she has fused with the power of the Book, but to no avail. They even attempt to communicate with her, telling her that she was tricked, but she cries and, as the Book, says that she is only carrying out the wishes of Hayate. The Book claims that Hayate, losing her friends, has been driven to near-madness in much the same manner as Precia Testarossa had at the death of Alicia prior to the first season. The girls try again, but the Book says that she is simply a tool and will not last long, but must carry out her master's wishes. She absorbs Fate into a dream from which she will never want to wake and reveals that Hayate is in much the same sort of dream. In the middle of this, 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, and 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. She takes control of her body again so that Nanoha can disable it and gives the Book a new name, "Reinforce". Hayate reprograms the entire Book, but something still remains inside that wants to continue its path of destruction and pain. She identifies it as a corrupted automatic defense program and that it caused the evil aspect of the Book in the first place. Nanoha, Fate, Hayate, Reinforce, Chrono, Arf and Yūno meet up inside the Book's pocket dimension, along with Signum, Shamal, Vita and Zafira, whom Hayate restores to life. They team up and, pooling all of their power, overpower the creature's defenses and teleport it into space, where the Asura fires the Arc-En-Ciel at it, destroying it. Reinforce believes that she has to be destroyed for the virus never to return, but Hayate refuses to let her go. Stating that she will always be with Hayate, Reinforce sacrifices herself, and asks Hayate not to give her name to the powerless remnant left by her destruction, but to the new weapon that Hayate creates to serve her.
In an epilogue that takes place six years later, Hayate is older, able-bodied, still living with the Wolkenritter, and working for the TSAB along with Fate and Nanoha. She sends a picture of herself and the Wolkenritter to Graham, who has retired and who promises to tell Hayate the truth when she is old enough to live on her own.
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