Losing everything he had at a very young age, Ciel makes a contract with a demon he calls Sebastian Michaelis to gain his revenge in exchange for his soul. He is portrayed to be emotionless and driven to achieve his goal by any means necessary. However, as the series progresses, he confesses to being the way he is so that he will not lose anything, or rather, anyone important to him ever again.
I believe there's a similar manga illustration that shows Ciel holding a skull. I can't remember which chapter that was though. It makes me wonder if that thing the cosplayer is holding is a real skull.
EDIT: Oh, the art similar to this cosplay was first seen on Kuroshitsuji Chapter 13. Thanks to the anon who left a comment!