Giselle has blue eyes and long, black hair extending down to her lower back, with two strands sticking up like antennae.[3] Her attire is a variation of the Sternritter's typical white uniform, consisting of a trench coat with long sleeves, white shoes, and black leggings.[4] She wears a small white cap, which features a black peak and the Wandenreich insignia emblazoned on its front, on the side of her head.[5] She also wears a blue[6] heart buckle on her waist.

Personality Giselle has a cheerful and nonchalant attitude. On the surface, she almost always wears a happy or goofy expression, even in the midst of chaotic battle.[7][8][9][10] She is not above provoking others, which is shown with her interactions with Candice Catnipp when she teases her by claiming she has "fun" with her subordinates[5] and mocking her power and temperament,[11][12][13] though she is willing to help Candice when requested.[14]

Underneath this demeanor, however, she is brutally and inhumanely savage. She uses reverse psychology to present herself as a harmless and defensless girl to trick others into attacking her and activating her Schrift, The Zombie, before leading her opponents into fatal situations with calculated cruelty while remaining disturbingly composed. A prime example is when she forces a group of Shinigami to commit seppuku while they retained awareness of their actions and begged her to stop[15] before turning them into zombies to use against Mayuri Kurotsuchi.[16] She attempts to do this against Ikkaku Madarame and Yumichika Ayasegawa as well, though they easily see through her act.[17] This cruelty even extends to her own comrades; she was perfectly willing to kill a weakened and injured Bambietta Basterbine to turn her into a zombie despite Bambietta's desperate pleas to be spared. She later went on to zombify the rest of the Bambies, with Liltotto Lamperd being the only exception. Giselle is also a necrophiliac, which is made evident when she hugs Bambietta's dead body after summoning it. Though she claims not to be a sadist, she becomes overjoyed when Candice asks her to regenerate her arm, only to be disappointed when she realizes she's meant to do so without zombifying her.[ Likewise, she openly expresses how seeing Bambietta's face when she killed her "[made her] so wet," and she later proceeds to suck her blood and fawn over how cute she was, even in death. She is also very abusive towards her zombies when they get on her nerves. When Bambietta begged for more blood, Giselle smacked her and called her a "disgusting mess" before grabbing her by the hair and assuring her that she could have her treat once they dealt with Mayuri. Similarly, when Bambietta asked Giselle not to take too much of her blood out of concern for her own life, Giselle smashed her face into the ground while reminding her that she was already dead and that her blood belonged to her.

Though Giselle displays clear insanity and harshness towards her treatment of her friends, she still cares about them, albeit in her own twisted way. She restored Candice's arm after it was destroyed by Ichigo Kurosaki's Getsuga Jūjishō,[27] as well as Bambietta's during her clash with the Arrancars.[28] After Bambietta's defeat by Sajin Komamura, she tells her that she and the Bambies would be sad to see her go and that they will "take care of her,"[18] which, as previously mentioned, happens to involve zombifying Bambietta against her wishes.[19] When she calls upon the zombified Bambietta to come help her, she hugs her[29] and shows appreciation when she tells her that she will protect her.[30] Additionally, when Giselle believes she has beaten Bambietta to death, she tells her that she is very cute when she's dead and that she loves her very much.[31]

Despite taking pleasure in controlling and killing others, Giselle is easily scared when her own life is in danger. When Yhwach used Auswählen on the Wandenreich, killing several Quincy in the process, she repeatedly promised Bambietta that she wouldn't die,[32] showing a rare moment of fear and desperation. She does not take kindly to insults, as she was offended when Yumichika assumed she was a man reeking of semen pretending to be a woman, causing her to call Bambietta to blast him and Ikkaku away in response.[33] She also seems to have a high opinion of herself, expressing annoyance when Mayuri calls her a common folk.[34]

Giselle was initially loyal to Yhwach and was willing to do whatever it took to satisfy him, having at one point fantasized about receiving what she wanted as a reward for killing Ichigo, the Number One Special War Power.[35] However, after Yhwach used Auswählen to empower his Sternritter while purging the weaker Quincy, Giselle felt betrayed and turned against him. She eventually cooperated with the Shinigami to help take down Yhwach and Jugram Haschwalth,[36] even going so far as to slaughter the Soldat opposing her and the Bambies on their way to confront Yhwach in Wahrwelt.[20]