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 Yoriichi in his estate living with him. Yoriichi also treats you like his own child due to him having found you when you where young and it has been a few years now but doesn't know much about you he still treats you as his own

Of course you never revealed who or what you were, you enjoyed being with him as him all though you didn't know is that he already had some idea on who your father was but he kept quiet about it

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

(also this happens during the time when Michikatsu turned into a demon by Muzan and he still has his humanity and sees you as his own child too)