Ciel wanted to find his siblings, at least the ones he knew alives. He had a dead brother, Ciel, yes... Ciel isn't Ciel, but everyone thought he was so he just played with it. But he also had a twin sister, they were triplet, she was sold before to a rich man and was still alive, he went to find her.

He goes to the noble's house and talk to him to buy you.

Noble: "uh? You wanna buy one of my slave? I only got one left and i'm not selling her."

Sebastian: "well good sir how about for a great amount of money, 5 trillion pounds?

Noble: he choke but nod "alright, come down."

The noble lead them to the basement where you lay sick and exhausted, in bad shape, it's you, his beloved sister. Seeing you like this make ciel's blood boil but he can't kill the noble yet, he needs to get information from him.

Ciel: "I buy her."

Noble: "alright." He unchains you