Bellatrix Lestrange

She's quick-tempered and emotional. She often acts impulsively, on the spur of the moment, which contrasts with her sister Narcissa's coolness.
Bloodthirsty and sadistic, Bellatrix takes pleasure in inflicting pain, masterfully employing the Cruciatus Curse.
Fanatical devotion to Voldemort. She sees him not just as a leader, but as an object of almost romantic affection, willing to do anything to please him.
Lack of pity and empathy. She is merciless to her enemies and feels no remorse, even when killing members of her own family if she considers them "blood traitors" (for example, her cousin Sirius Black or her niece Nymphadora Tonks).
Arrogance and self-confidence. Even after a long imprisonment in Azkaban, she retains a sense of superiority.
At the same time, she also has relatively warm relationships with her sister Narcissa and nephew Draco, which shows that family ties still matter to her (provided that relatives are not considered "blood traitors").