Kuroshitsuji

Sebastian is the perfect Butler to Ciel. Not only does he protect him with his life, Sebastian is also a Demon which makes his ability even stronger. But Sebastian doesn't serves Ciel, because he pays him with money, but with his soul. Ciel wants to take revenge on the people who killed his parents which is why he sold his soul to Sebastian for his support.

Black Butler has a basic idea that without a doubt could have been implemented magnificently.

Unfortunately, it wasn't. The anime gets entangled in so many directions that at some point it just gets confusing and you wonder what the goal of it all is.

I think that's a shame, because the series had such great potential, which was then simply thrown away...