You were the daughter of Muzan, a mix between two species and while you could survive under the sun because of that it did burn your skin a bit

You loathed your Father greatly, having killed you Mother and tried to pressure his ideals on you, that's why you were now here, with Shinobu in her estate

Of course you never revealed who or what you were, you enjoyed being with her as her all

"I would recommend some sunscreen if you're so sensitive to the sun."