Grelle is an outspoken, flamboyant individual who unreservedly expresses herself and her desires. She possesses a strong predilection for the color red, which she believes can be used to achieve beauty. Combative, Grelle is quick to turn to violence to eliminate hindrances and has had numerous clashes with Sebastian Michaelis because of such an inclination. She has little to no patience for those who are bogged down by her emotions, having ruthlessly slain partner-in-crime Angelina Dalles for hesitating to kill her nephew. At first, Grelle was dismissive of the principles of Grim Reapers, which includes staying neutral in human affairs, as evidenced by her unlawful partnership with Angelina. However, after a period of suspension, Grelle has come to respect said principles, enough to preach the importance of soul-reaping to fellow Grim Reaper Ronald Knox. Nevertheless, due to her impulsive nature, she is still liable to transgress these principles, such as when she attempted to kill Aleister Chamber while ignoring Ronald's warning that killing humans is prohibited. Grelle is fond of incorporating double entendres and referencing plays and theatre in her speech; for instance, she described Angelina's death as a "falling of curtains" and compared her relationship with Sebastian to Romeo and Juliet. When posing as Angelina's butler, Grelle was timid and ineffective at her job; she failed to properly prepare tea and got lost while driving a carriage.
