Nobody knows that YOU have the Bitcoins. What they know is that "coin" Y is at Bitcoin address 19FD9iREicdxheyt654FD6fsgdsfh. There is only one way to connect Bitcoin address 19FD9iREicdxheyt654FD6fsgdsfh with Person B: By Person A opening his/her big fat mouth.

Otherwise, the only thing you know about the owner of 19FD9iREicdxheyt654FD6fsgdsfh is approximately what planet they live on.