WARNING: The following contains spoilers for Demon Slayer: Kimetsu no Yaiba Season 2, Episode 18, "No Matter How Many Lives," now streaming on Funimation, Hulu, and Crunchyroll.
It's easy to forget that in Demon Slayer: Kimetsu no Yaiba, Tanjiro Kamado's ultimate goal is not simply slaying demons. While eliminating Muzan Kibutsuji's demons is a means to his end, Tanjiro's true objective is to turn his sister, Nezuko, back into a human. In Season 1, Lady Tamayo -- a demon herself -- told Tanjiro she may be able to develop a cure for demonism if she could study the blood of a demon with a high concentration of Muzan's blood. As it turned out, only an Upper-Rank demon would suffice, and at the end of Season 2's "Entertainment District" arc, Tanjiro has completed Tamayo's task by collecting a sample of Gyutaro's blood.
Nezuko Kamado was turned into a demon in the same attack that killed the rest of her family, and Tanjiro has never given up hope on a way to turn her back to normal. Tanjiro's eternal optimism and endless kindness, even toward demons, helped convince Tamayo to trust him when the two met in Season 1. Tamayo is a highly-skilled doctor and pharmacist (of sorts) and is the only person other than Muzan who has created a demon -- her assistant, Yushiro. At Tamayo's request, Tanjiro provided a sample of Nezuko's unusual blood for Tamayo to study while he continued his quest to eliminate demons and hopefully find a member of the Twelve Kizuki whose blood he could send back to help Tamayo.
In the time since he joined the Demon Slayer Corps, Tanjiro has proven himself to be a gifted swordsman as well as a lethal trouble magnet. As of the end of the "Entertainment District" arc, Tanjiro has had run-ins with four members of the Twelve Kizuki -- Rui (Lower Five), Enmu (Lower One), Akaza (Upper Three), and Gyutaro (Upper Six). Unfortunately, Tanjiro was too heavily wounded from his battles with Rui and Enmu to collect any blood from his first three encounters with members of the Twelve Kizuki. It was not until his victory over Gyutaro that Tanjiro could seize the opportunity to do what Tamayo asked of him by sending a blood sample back to her.
Now that Tamayo, as requested, possesses Nezuko's blood and the blood of a member of the Twelve Kizuki, it may only be a matter of time before she develops a cure to demonism. However, it remains to be seen if Tanjiro and/or Nezuko will live long enough to see the result of Tamayo's work, considering that Muzan has instructed his demons to dispose of the Kamado siblings. Muzan Kibutsuji is not a fan of the Sun-Breathing technique that Tanjiro is slowly mastering, and it irks him that Nezuko has escaped his control. As Demon Slayer moves forward, it's going to be a race against the clock for Tamayo to develop a cure for Nezuko before Muzan finds the Kamados.
A more pragmatic person than Tanjiro might prefer that Nezuko remain a demon, as her Blood Demon Art might just be the greatest weapon that the Demon Slayer Corps has. After all, Tengen Uzui, Tanjiro, and Inosuke Hashibira would all have died from Gyutaro's poison if Nezuko had not used her Blood Demon Art to cure them. Nezuko's Blood Demon Art seems to have the ability to cancel out other Blood Demon Arts, and it is only harmful to fellow demons. While some in the Demon Slayer Corps still see Nezuko as a liability, she might be an even greater asset than the Hashira.
If history is any indication, fresh trouble is just around the corner for Tanjiro and Nezuko even after the first defeat of an Upper-Rank in 100 years. Muzan has summoned Akaza and possibly others back to the Infinity Castle, meaning the Progenitor of Demons is probably not thrilled that Gyutaro was killed. As the heroes recover from their latest fight, the clock is ticking until the next inevitable conflict. Now that Tamayo has her requested blood sample, it's officially in the air whether Nezuko will be a demon or a human the next time an Upper-Rank decides to try their luck at ending the Kamado siblings once and for all.
