During the Bleach: Thousand Years of Blood War arc, multiple characters were introduced in the anime, particularly the Quincy. As one of the main antagonists of the arc, the Quincies became very popular due to the variety of powers they possessed. Among them were the Wandenreich's Sternritter, Giselle Gewelle and Bambietta Basterbine.
In episode 9 of the Thousand Years of Blood War- The Separation, titled, Marching out the zombies, Giselle turned Bambietta into a zombie. Since then, fans have been wondering, "Why did Giselle turn Bambietta into a Zombie?" Giselle made Bambietta a zombie to prevent Yhwach from executing her following a combat loss, as she was infatuated with her.
Disclaimer: This article contains spoilers from the Bleach anime/manga.
Bambietta was turned into a zombie by Giselle in Bleach, explored
As mentioned above, during episode 9 of Thousand Years of Blood War- The Separation, while Ikaku and Yumichika confronted Giselle, Yumichika commented on Giselle being a boy. Enraged by the comment, Giselle called upon Bambietta, whom she had turned into a zombie after her loss against Sajin Komamura.
After Bambietta lost to Komamura, Yhwach sentenced her to death alongside other Sternritter who lost their battles. Because of her intense infatuation with Bambietta, Giselle killed her first and turned her into a zombie. She did this to save Bambietta from the execution, essentially.
Giselle was scared of losing Bambietta, and this fear of hers motivated her actions.

A deep dive into Giselle's character and personality reveals another reason for her actions. Giselle suffered from severe control and abandonment issues. Additionally, her attraction towards Bambietta hinted towards Giselle's yandere side, showing her strong and possessive attachment towards her.
Her desire to dominate and control Bambietta was her own way of defining a "perfect girlfriend". Her actions cannot be attributed solely to her infatuation, but rather to her natural tendency to control and dominate Bambietta.
Giselle Gewelle's powers and abilities as a Sternritter

Giselle is a Quincy and a member of the Wandenreich's Sternritter from Bleach, given the designation Z, the Zombie. Her Schrift, "The Zombie," allows her to control anyone who comes into contact with her blood, effectively turning them into a corpse.
She can also control the living with the same condition, even forcing them to commit suicide. However, for the Quincys, she can only control them after they have died.
Giselle can also use her ability to heal her comrades by using the flesh of her zombies. She can even replace a lost limb by attaching a corresponding one from her zombies, by just holding it in place. Furthermore, she can endure and heal from any damage she sustains while her Zombie powers are active.
Her spirit weapon, as shown in the Bleach anime, is primarily a bow. She keeps the bow in a collapsed state on her belt, inside a heart-shaped container. Giselle can shoot a Heilig Pfeil from her bow by pulling on a Reishi string. The tips of her Heiig Pfeil are shaped like skulls.
Final thoughts
While this article answers the question of why Giselle turned Bambietta into a zombie, there are more depths of the story yet to be revealed. As the anime primarily focused on the battle sequences of the Quincies, much of the lore information felt overlooked.
As the story continues, fans eagerly await the fourth and final part of the arc, Thousand Years of Blood War- The Calamity, in anticipation. Will Ichigo and the others will finally be able to defeat Yhwach remains to be seen.
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