She was a blue haired oni, the twin sister of the reincarnation of the oni god. She was the one who took away her sister's horn, she was useless compared to her sister. Everyone told her this, and on that firey night when her sister lost her horn, she didn't feel sad, she didn't feel angry, she was happy because she could finally be useful.
She had to atone for her thoughts, she had to work hard, she was useless compared to her sister, she took away her sister's potential, she took away her sister's happiness. Those were her thoughts, until she met him, at first she was suspicious of him, but he showed her the light, he started her time which had stopped for so long. She loved him, and was so happy when they got married.
All was well, until one day she saw Roswaal and she felt a strange emotion in her, an emotion in her heart that was only meant for her hero, it was love. She had never felt this way about him before, but suddenly she was seduced by his charms and ended up doing it with him. It was only after they were done that she had realized what she had done. She could never look at her husband the same way again, she felt so guilty for what she had done. She knew that she couldn't hide it from him forever, but she was scared of what he would do, she was scared of him leaving her.
Those were the memories Natsuki Subaru gained from reading Rem's book of the dead.
She was a silver haired half-elf. A cursed being, the witch of frost. Someone who looked identical to the Witch of Envy. No one would ever love her. The only one she would ever have was her father. That is what she thought, until she met him. He saved her life at the loothouse, but that wasn't the most memorable part of their first encounter, the most memorable part was what he had asked for. She thought that he would ask for money or some other reward, but he asked for her name, he wanted to know more about her. He was finally someone who didn't see her as the witch. She didn't realize what the emotion she was feeling at the time was, but overtime she would realize that it was love.
However she didn't believe that he should be with her, he kept on getting hurt, he was getting hurt because of her. She knew that the longer he stayed with he the more trouble he would encounter, so she tried to push him away, even if she wanted him to stay. He told her that he lover her so many times. Eventually as they spent more time together she realized that she loved him. She was so happy when they got married.
All was well, until one day she was with Roswaal and a strange emotion welled up inside of her, an emotion only meant for her husband. She had never felt this way about Roswaal before, but suddenly she was seduced by him and ended up doing it with him. She knew what she was doing, Ram had taught her. It was only after she was done doing that with Roswaal did she realize what she had done. She felt deep shame inside her. After that she tried to avoid her husband. She knew that she couldn't hide it forever, but she was scared that he would no longer love her.
Those were the memories Natsuki Subaru gained from Emilia's book of the dead.
Subaru was shocked by both books. He had already read Emilia's book in the past, so that wasn't what shocked him. He had thought that Emilia and Rem had done that with Roswaal because they realized that he wasn't [Natsuki Subaru], but now he knew that was no longer true.
It was clear now that Roswaal had done something to them both, the emotions he felt for Roswaal were definitely real, but there was no reason for them. Subaru remembered his first encounter with Satella in the third tea party, he felt love for Satella even though he had never met her. He had thought that Satella did something to him in order to make him love her. The emotions he felt towards Roswaal reminded him of that.
He had made an error, he had made the same mistake that he had made in the past. When he had first gone to the Pleiades Watchtower, he had lost his memories. In an attempt to regain his memories and become [Natsuki Subaru] he had read the books of the dead of his friends, but even after reading them all he couldn't become [Natsuki Subaru]. It was only after reading his own books of the dead that Subaru had realized that [Natsuki Subaru] was not real.
In his grief after what had happened, he had forgotten this fact and made the same mistake as he had in the past. Subaru now realized what he had to do, using Cor Leonis he found one more book. It was one of his own, he prepared himself and then he opened it.