Sidney Black is the Black Rabbit and the Prime Minister of the Castle of Diamond. Unlike Peter, he hates Alice. He is high-strung and gets irritated very easily. He hates the colour white and wants to paint everything black. This means that he hates the Queen of Diamond's love of white but he is still her faithful subordinate. He's very worried that all the stress will give him grey (in Japanese, white) hairs.

Like Peter White has white hair, Sidney Black has short black hair. He has heterochromia iridum, with one red eye, covered by a monocle and a black, slightly grey eye. He appears to be a lop ear rabbit, since his ears are always down compared to the other rabbits in the game. He wears a small top hat with the same flower on his chest. He also has one glass lens for his red eye only. The outfit he is usually seen wearing—a white tuxedo shirt under a yellow, pink, and black vest with a large pink bow in the back, black shorts, and long black boots—is of Crysta's choosing. He has been shown wearing much more relaxed and all black clothes when younger. His personal style is probably a lot simpler, in Alice's opinion.

He came from a family of black rabbits, but, due to his red eye, they abandoned him, because his nearly blind red eyes was a deformity and a weakness. Crysta accepted him when his family left him and took him in, both because she wants him as part of her collection and because she loves rabbits. As he has two different colored eyes, he is unique, and Crysta wants him in her collection of frozen things for this reason.

His insistence that he truly was a black rabbit and that he wasn't "nothing" lead him to make a deal with Joker, and his hatred of the White Rabbit is because of, or at least includes, Peter's apparent lack of emotions, while Sidney himself feels strong emotions.
He's a very high strung person who takes everything seriously, often too much so. As cruel as Peter White when dealing with others, whether they be subordinates or enemies, he is not above shooting a person simply because they did not do their job correctly or they said something he didn't approve of. A hard worker, Sidney pushes his subordinates to work almost as hard as him, yet he'll also force them to take breaks so that they don't work themselves too hard. He's the sort to yell and insult a person when he's worried about them and will demonstrate concern over the smallest things. Sidney accurately falls under the category of "tsundere," as he is harsh and downright rude most of the time yet being gentle and affectionate when he feels the need.

Unlike the White Rabbit, Sidney doesn't have a hatred of germs, as shown by such incidents as his brushing sweat from Alice's forehead and eating a bite of cake off of her fork. He claims that he hates Peter White above all other people and is incredibly competitive of him, to the point where he will do something simply because Peter did it.

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