William Macbeth

A fairly talkative person with a sweet and caring nature, he is also a rather awkward individual. Upon meeting Leonardo for the first time, he readily introduces himself and acts in a relaxed and friendly manner.

During Leo and Black’s lunch together, it is established that the two have a lot in common, especially when it comes to having low self-esteem. Black is shown to feel rather insecure about himself and his abilities, showing no hesitation in talking down on himself when recounting memories of being bullied in his younger years. Along with this, Black is also shown to be awkward and clumsy when he spills his drink all over himself in front of Leo, and is shown to panic in the embarrassing situation in an exaggerated manner.

He is White's older brother. Black is incredibly protective of his younger twin sister White, as seen in multiple occasions throughout the series, especially when he is seen desperately trying to keep the King of Despair from involving her in his plans. As children, he and White were raised lovingly by their parents, however Black began to develop feelings of being lesser than his family due to the fact he saw himself as emotionally weak. He came to this conclusion when he and White were making mud balls together when they were very little, and when White made better a mud ball than him he used his psychic powers to burst the ball out of frustration. He expected White to start crying, but instead she just went straight to making another mud ball, and Black ended up being the one crying. Due to this, Black felt as though he was much weaker than his sister, and showed great self-restraint in distancing himself from his powers as much as possible in order to try to better himself as a person, even going as far as letting bullies beat him up without fighting back.

(Source: Kekkai Sensen Wiki)