Gyutaro’s entrance was one of the best and scariest in Demon Slayer, but why was he in Daki’s body in the first place?
Demon Slayer: Kimetsu no Yaiba’s biggest surprise in the “Entertainment District” arc was Gyutaro escaping from Daki’s body to battle Tengen Uzui. Even though Tengen knew Daki was much too weak to be the demon he was seeking for, let alone a High Rank, Gyutaro’s frightening and ugly arrival caught him off guard. Although it soon emerged that Gyutaro was the true High Ranking Six, why he was in Daki’s body was never clarified.
Gyutaro and Daki were siblings when they were human, and her name was Ume. They lived in poverty until Gyutaro found her fighting abilities, and Ume began to draw attention to her beauty. Ume has never been ashamed to ask her older brother for assistance, and Gyutaro is quite proud of her. He regarded her as his pride and pleasure, and was heartbroken when she died in a fire while he was gone. Once they became demons, Gyutaro apparently discovered a way to never risk being separated from Ume again.
Gyutaro and Ume’s lives became inextricably entangled when the former High-Rank Six discovered them and turned them into demons. Daki cannot be killed while Gyutaro is in his body, and neither can be killed once he leaves unless both are decapitated at the same time. Tengen Uzui, on the other hand, believed that if Gyutaro absorbed Daki and then was decapitated, they would both die, which Gyutaro did not verify. Daki was far closer to death than Gyutaro when they were both changed into demons, hence he appears to have become the two’s main unit.
Gyutaro’s ability to exist inside Daki’s body apparently not only serves to ensure that they are never separated, but it is also the safest state of being for her. Keeping his sister safe is Gyutaro’s only goal in life, so he seems content to stay in her body and let her do whatever she wants until she needs him. Gyutaro cares for Daki above all else, even when he is forced to leave her body to defend her. Gyutaro even gives Daki one of his eyes, not only to provide her with vitality, but also so he can keep an eye on her as they fight apart.
Gyutaro asserted to Tengen that he and Daki were two people in one, and it became apparent that he wasn’t just talking about their power and Upper Six status, but also their bond. Gyutaro’s sister Ume was the light of his life when they were children; having such a beautiful sister removed his inferiority issue and he grew proud of his hideous appearance. Gyutaro told Daki that they would never be separated as they kept each other warm through the cold. In the afterlife, his promise was fulfilled when a childish Ume opted to accompany Gyutaro to Hell rather than embrace the light without him.
Tanjiro Kamado, the protagonist of Demon Slayer, observed a curious resemblance between his connection with Nezuko and Gyutaro and Daki’s. Gyutaro would go to any length for Daki as siblings who love each other unconditionally. Gyutaro refused to let Ume die, just as Tanjiro refused to let Nezuko die since she was a demon. And if giving up her own life to exist inside Daki’s body helps keep her safe, Gyutaro had no problem remaining idle while his sister lived the life he thought she always should have had.