*Manga spoilers ahead for chapter 126 onwards*
Nezuko is called ‘The Chosen Demon’ because she is able to walk in sunlight. But why is she the only demon to ever exist with an immunity to sunlight?
Well, it’s most likely because of her lineage.
It’s implied throughout the series that Tanjiro and his family are connected to the first Breath of the Sun user, who is also the first ever breath user. They all wear the same earrings, have a similar scar on their forehead and look similar. Plus, someone like Tanjiro with ‘red hair’ with ‘red eyes’ is said to be a ‘child of brightness’ and this is something considered lucky in families that work in fire, and are consistent traits with those who previously had potential to be a Breath of the Sun user. Because of this connection Nezuko might have a special bodily constitution that allows her to walk in the sun.
However, Nezuko has been burned before by the sun. Why did she only develop this ability as the manga progressed?
Perhaps the reason she can withstand sunlight then is because she protects humans instead of eating them. She got burned in the sun to save Tanjiro, and this act of selflessness and self-sacrifice for her brother is something that has never occurred in another demon – so she might have gained this ability.
Or maybe it’s because she is the only demon to never have consumed human flesh or blood and instead sleeps to restore energy. This would mean that all demons can withstand sunlight, but because of their desire for human flesh and blood they can’t. (How ironic this would be for Muzan.)
Nezuko could be the only demon to not consume human flesh or blood because she is the only known demon who didn’t have a tragic past. Nezuko’s trauma of her family dying occurred when she was turned into a demon, while other demons had something in their past like illness or abusive parents that influenced why Muzan took advantage of them. This difference could be why she maintained human emotions, why she was able to be influenced by Urokodaki during her 2 years of sleep to protect humans, and why could sleep instead of eating humans. Leading to her gaining the ability to walk in the sun.
Does anyone have any of their own theories as to why Nezuko can withstand sunlight? What theory makes the most sense to you?