Bellatrix Lestrange:

She's of course quite evil and makes not much sense except in the sense that she loves the dark lord more than anyone. She is most loyal to him in all ways and show it to him. She is quite crazy almost all the time and will express hate and dislike towards anyone who is a muggle-born/half-blood/blood traitor. She won't take any hesitation to kill anyone who stands in her way. She often likes to torture people and kill them. She follows her master's commands without question.