Humans believed in the idea of reincarnation, and when humanity once thrived, there was a genre called "isekai." 2B had no real reason to learn about things unrelated to her objectives. She was a machine. She didn't have a soul. Someone upstairs thought otherwise. Her mission had not changed, even as a reborn human in a world inhabited by demons. Glory to mankind. —Fanfiction Dot Net summary
2B(a)DEMON SLAYER is a Crossover between Yoko Taro's NieR: Automata and Demon Slayer: Kimetsu no Yaiba, written by TheStrangerThatCameFromNowhere
, author of Fate/VRAINS fame.
Set after 2B's death in Route C, she awakens and finds herself reincarnated in Japan during the Taisho period, much to her shock and surprise. She has since lived the life of Yona, the daughter of Yukito and Kiyo, and lives according to how she is expected to live while dealing with the fact that she is now living in the body of a human being, much less in a time period when humanity was still thriving.
A day after the passing of her mother, who was suffering from tuberculosis, 2B encounters the Stone Hashira Gyomei Himejima, who is in search of a demon hiding in her village and discovers that the daughter of the village's butcher was found dead, seemingly with her face torn off. That same day, she encounters the enemies of humanity — the demons.
Following her father's death, having succumbed to his wounds while protecting her, 2B remembers her previous mission as an android commissioned by YorHa and decides to continue it by becoming a Demon Slayer.
The story can be read here and here.
Beware unmarked spoilers for the manga.
2B(a)DEMON SLAYER contains the following tropes:
- Action Girl: 2B, which is a given considering her track record. Even when she is reincarnated as a human, she is still capable of kicking ass.
- Actor Allusion: 2B briefly freezes when she first hears Tanjiro speak, due to him sounding like 9S. Both of whom were voiced by Natsuki Hanae.
- Adaptational Early Appearance: Gyomei Himejima, one of the Hashira, first appeared when Tanjiro was questioned by the Hashira in regards to his harboring a demon and debating what to do with him. He appears a few years ahead of schedule, as he is tasked with eliminating the demon hiding in 2B's village.
- Adaptational Wimp: 2B was one of the most skilled combatants within YorHa, and being an android, she could take hits that would have cripped the average human. Having been reincarnated as a human, whose bodies are significantly weaker and less durable than a machine, she realizes that she won't be capable of some of the feats she was able to do. Lampshaded in the first chapter, where she notes that whereas androids and machines could be repaired and even rebuilt, human lives were finite and couldn't be replaced.
- Adaptation Expansion: The Breath of the Moon. In the manga, the only known user was Kokushibo. Here, Kokushibo taught the technique to his students before he became a demon. When he did, the Demon Slayer Corps was not too keen on using a technique that was created by a Demon Slayer who became a demon, and as a result, the number of practitioners has lessened to the point that, by the time 2B is born, there are only ten people in the world who can use and teach the Moon Breathing, including her father Yukito.
- All of the Other Reindeer: The Breath of the Moon sword-style and anyone who uses it. Though the Demon Slayer Corps lost many of its records of the past, including information pertaining to the original demon slayers themselves, they do know that the Breath of the Moon was created by a demon slayer who became a demon. As a result, many of its members prefer not to use a style created by a traitor, and those who do practice it were ostracized. The sword style is so rarely used that, including 2B, there are currently only ten people who use the Breath of the Moon.
- Aura Vision: Tying with some Demon Slayers' enhanced senses like Tanjiro's excellent nose and Zenitsu's hearing, 2B can see a mist around certain individuals, though it appears to be a recently developed skill. Demons have a bloody mist around them, and her father had a silvery mist around him when he prepared to use a Breathing Technique.
Word of God states that this is a watered-down version of the "see-through world" used by those possessing the Demon Slayer's Mark. - Cassandra Truth: When she first learns about the demons, 2B doesn't think they exist, rationalizing that people claim a demon exists in order to explain unknown phenomenon and believes the "demons" are actually demented human beings. It isn't until she sees a demon for herself that she believes they exist.
- Difficult, but Awesome: The Breath of the Moon swordstyle, natch. It's one of the strongest Breaths used by the Demon Slayer Corps., but it is incredibly difficult to use properly if the fact that Isshin has 2B train to use Total Concentration: Constant instead of Total Concentration Breathing is of any indication.
- Expy: 2B's uncle, Isshin Ashina, is based off of the character of the same name from Sekiro: Shadows Die Twice.
- Foregone Conclusion: Confirmed by Word of God in Chapter 2 when answering reviews that 2B will become a Hashira.
- Future Imperfect: Not for the world in general, but the Demon Slayer Corps. Many of its old records have been lost, including the identity of the Breath of the Moon's creator (though they do know he betrayed his comrades by becoming a demon) and the Breath that was once used by the very first Demon Slayer.
- Handicapped Badass: Gyomei is blind, but this does very little to stop him and is one of the strongest demon slayers in the organization. 2B is a downplayed example in that she wears a blindfold in order to use her Aura Vision to its fullest extent.
- Heroic Lineage: 2B's grandfather was a former Demon Slayer, and the fact that Yukito was taught the Breath of the Moon technique means that he was either a Demon Slayer himself or was at least trained to become one. Her uncle Isshin was also a member, but at some point he retired and took up the life of a merchant.
- Heroic RRoD: When 2B attempts to use a Moon Breathing technique while she has only had a year's worth of training, the strain causes her incredible amounts of pain and leaves her completely helpless and exhausted afterwards.
- Heroic Sacrifice:
- When 2B is captured by a demon and attempts to use her as a bargaining chip, she tries to give up her life by asking Gyomei to attack, showing that she is more than willing to give up her own life if it means the demon will be killed.
- 2B's human father, Yukito, dies after saving his daughter from a demon who attempted to use her as a hostage by using the first form of the Breath of the Moon technique, which in turn pushed his body well past the limit and worsened his wounds.
- Humble Hero: 2B's uncle Isshin. According to Kagaya, he was one of the best Demon Slayers under his employ in the short time he's been the commander of the Corps and was easily a prime candidate for the position of Hashira. He ultimately turned down the position.
- Meaningful Echo: "Glory to mankind" was uttered by the members of YorHa as per their allegiance and loyalty to the human race while battling the machines on Earth. The words ring hollow, however, as humanity in 2B's time was extinct for years, meaning their loyalty was to people who were never there in the first place. After the death of her father and encountering demons for the first time, 2B utters "Glory... to mankind" to symbolize her return to the battlefield and her duty to protect humanity from the demons.
- Meaningful Name: 2B's family name is "Tsukiko." Tsuki means moon, which ties to both her previous life as an android whose base of operations was situated on the Earth's moon and her choice of sword style, the Breath of the Moon, which was created by one of the Upper Twelve Kizuki. Ko means child. In translation, "moon child."
- Papa Wolf: Yukito. He's suffering from a terrible injury, but he gives up his life to kill the demon trying to use his daughter as a hostage.
- Pun-Based Title: "2B(a)DEMON SLAYER." As in, "to be a demon slayer." On the other hand, the name of the story is clever as 2B originally planned on living a life that was expected of her in Taisho period Japan, only for the death of her father to set her on the course of remembering why she was built in the first place and identifying that goal with the Demon Slayers.
- Red Herring: The big sign that a demon appeared in 2B's village is that the daughter of the butcher, Takeomi, was found dead with her face having been ripped off. Gyomei confronts the demon, the village elder, who he found feasting on the maid. Gyomei believes him to be the culprit... only for the village elder to tell him that he wasn't the one who killed Takeomi, but he did try to target her. This, coupled with the fact that 2B sees a red mist in the village, tells Gyomei that the village elder is not the only demon in the village.
- Sink or Swim Mentor: Isshin's first real test for 2B is to hunt down and kill a demon hiding in Shanghai, when she has only had a year's worth of training. 2B does note that Isshin was close by when she killed the demon, so odds are she wasn't in any real danger to begin with.
- Training from Hell: What Isshin subjects a six-year-old 2B to. Justified since the Breath of the Moon is incredibly difficult to use otherwise without peak human physical conditioning, even requiring Total Concentration: Focused to use properly.
- Troubling Unchildlike Behavior: 2B. Gyomei notes that she is very wise for her age, especially when she states that he should have killed her along with the demon in order to prevent it from causing anymore casualties, as her life would be inconsequential to the lives of hundreds more. Justified, though, as 2B was an android fighting a war that went on for who knows how long and was reincarnated in the body of a child.
- Truth in Television: 2B's early life as Yona. Being reincarnated as a human, she intended to live a life of a self-dependant and courteous woman who obeyed her father as a child, then her husband when she was to be wed, then her son when she was a mother. This is actually what was expected of women in early Japan.
- Wham Line: Uttered in the first chapter, which demonstrates a certain Breath technique that was only used by a demon in the manga being used by a human.Yukito: Tsuki no Kokyū, Hachi no Kata: Getsuryū Rinbi (Breath of the Moon, Eighth Form: Moon-Dragon Ringtail).
- What Measure Is a Non-Human?: Discussed by 2B as she wonders why she, an android who probably doesn't have a soul, was reincarnated into a world where humanity was still thriving.
