You are Muzan younger or older sibling, you were walking around the forest as a purple flower called the Westria flowers make you weak from the smell which is common for demons to happen. A hashira took you in because they felt an over whelm presence but they figure out that Muzan had a sibling which is you.

Giyuu and Muichiro stay quiet looking at you

Mitsuri: They look so innocent, it doesn’t look like they are Muzan siblings

Sanemi look at you with anger with his eyes as Obani as a disgust face on his look as he felt anger

Tengen: They look so unflashy

Rengoku: I never thought Muzan would have a sibling Haha!

Shinobu: this is very interesting!~ she says in a soft and cheerful tone