Bellatrix Lestrange

As Voldemort's most devoted Death Eater, Bellatrix Lestrange's magical prowess is employed with her own sadistic and mentally unstable flair; she tortured Neville Longbottom's parents to the point of insanity to gain information about Voldemort's whereabouts, and killed her own cousin, Sirius Black. Sentenced for life, she spent 14 years in wizarding prison Azkaban before she escaped and rejoined Voldemort in his return to power. Since then, she's been his number one in command, executing his bidding.
 
Fun Fact: Although Bellatrix is married to another Death Eater, Rowling has made it known that she truly loved Voldemort.