Well, in the 'Black Butler Book of Circus side story', it's a rather interesting plot. The circus setting is full of unique characters. Ciel is on some sort of investigation related to the circus. Sebastian, as his butler, uses his extraordinary abilities to assist. There are elements of danger, as there are likely some unsavory things going on behind the scenes of the circus's seemingly jolly facade. And throughout the story, we see the complex relationship between Ciel and Sebastian play out in this circus - themed environment.
Well, in this side story, the circus serves as a backdrop filled with all sorts of interesting characters. The story delves into the lives of the circus performers, and there are often sinister undertones. Sebastian and Ciel get involved with the circus troupe for various reasons, like investigating something strange. The story is full of twists and turns as they try to uncover the truth behind the circus's activities.
Overall, it adheres pretty well to the manga. There might be some minor tweaks for the sake of animation, but the core story remains intact. The visuals and story progression match up nicely.
The main characters are Ciel and Sebastian. Ciel is the young master who is always in search of something and Sebastian is his butler who has extraordinary powers and is very loyal to Ciel.
The main characters include Ciel Phantomhive and his butler Sebastian Michaelis. Also, there are various characters from the circus troupe such as Joker, Beast, and others.
Definitely not. The Black Butler series has a set of established themes such as the supernatural elements, the Victorian - era setting, and the complex character relationships. The idea of a 'leashed slave story' is completely foreign to the official canon and is likely a disturbing creation outside of the proper series. It would go against the overall tone and moral framework of the official work.