He was surrounded by bloody situations most of the time, and there was no warmth at all.
Other than Alex, whom he had just saved, Sebastian really did not have anybody that was close to him and addressed him endearingly as mister like Kingston.
There were none abroad. There were also none back in his country. At that time, Sebastian's half-brothers saw no difference between him and a beggar.
Therefore, when Kingston tried to let himself stay by Sebastian's side by addressing him as mister, he