Sebastian is Ciel's father. Before Ciel was born, when he was Vincent, he made a contract with a demon to protect his son. When the Phantomhive manor burned down, Vincent gave his soul to save Ciel.

Author Note
Again, I do not know if this is true or not. The top half of the picture is Sebastian Michaelis and the bottom picture is Vincent Phantomhive. Just in case you didn't know that already.