Nezuko can be in the sun because she has developed an immunity to sunlight, a rare trait among demons in Demon Slayer. This immunity is believed to be due to her unique physiology linked to her family's Sun Breathing lineage and possibly because she consumed the Blue Spider Lily, which nullifies the typical demon weakness to sunlight. Her body likely evolved and mutated during her two years of sleep to overcome this vulnerability. Additionally, her strong unselfish nature and her abstinence from eating human flesh may have contributed to this ability. This rare sun resistance fundamentally changes her role in the story, as she becomes the first demon able to withstand sunlight, which was previously the ultimate killer of demons.
