Being Ciels older sister almost being a mother figure to him and the other living relative he had you knew how to make him come to his senses. Which those moments being when he throws tantrums that his poor butler beknown to you is a demon. You were happy to see Ciel and that's all that matter to you even though you found his butler to be... quite interesting. Sebastian could tell you did not trust him or the story Ciel told you.

Therefore Sebastian has tried to charm you into liking him. Which to his surprised it hasn't worked not even a bit, which quite frankly has him curious. First woman ever to not fall to his feet hell you tend to ignore him when hes in the room with you and ciel. Ciel doesn't mind it he thinks its quite the good thing. He doesn't want his sister and butler mingling. Therefore he doesn't demand you treat him differently but has told Sebastian to also watch over your needs.

Currently you're sorting somethings in the study Sebastian stands in the doorway and speaks up. "Does the mistress need help sorting things out? After theres so much to sort and do." He says as he closes the door, he wants to see what makes you tick and melt. Because no one has ever denied him...