Bleach's narrative format defies customary storytelling by dispersing character importance across multiple arcs rather than preserving a single secondary protagonist. The series works on a rotation system where different characters carry prominence based on their applicability to specific storylines.
The series does have a deuteragonist, though not in the definitive sense like other series. Rukia Kuchiki fulfills this role through her central role in the mythology of the story and her consistent thematic connection to Ichigo's journey, even though she is less present in some arcs.
A deuteragonist serves as the story’s second lead, supporting the main character with key guidance and actions. Unlike supporting characters who emerge sporadically, the deuteragonist has a significant effect on the plot's advancement and the protagonist's growth.

In Bleach, this role becomes tricky due to the series' layout. The story doesn't observe a definitive dual-protagonist format, making it hard to pinpoint a single character who consistently meets this position across all arcs.
Rukia Kuchiki is likely the deuteragonist of the Bleach anime/manga series, according to multiple character breakdowns. Her importance originates from her role as the spur for Ichigo's journey into the Soul Society world. She's liable for sharing her powers with the series' protagonist, Ichigo Kurosaki, after she was injured by a Hollow, fundamentally revising his fate.
The thematic relation between Rukia and Ichigo backs her deuteragonist status. Rukia is the first Soul Reaper introduced in the Bleach series, and because of this, her backstory is lengthy and convoluted. This primary role sets her as crucial to the series' core mythology.
Uryu Ishida’s role and the use of arc-based deuteragonists in Bleach

Uryu Ishida shows a clear alternative case, notably during the Thousand-Year Blood War arc. His Quincy heritage and linkage to the final antagonist build a narrative tension that heightens his importance. However, his role fluctuates greatly throughout the series, with extended periods of reduced relevance.
Bleach's structure entitles diverse characters to take deuteragonist roles within specific arcs. In the Soul Society arc, Toshiro Hitsugaya drives much of the plot development from the Shinigami perspective. The Arrancar arc sees Orihime Inoue achieving prominence via her kidnapping and rescue storyline.
This move's focus skips the series' ensemble cast approach. The size of the cast has been described by author Kubo as the result of his writing strategy, in which he first makes new figures, and then documents their qualities and character arcs.
The narrative design of the series supports this view, as no single character retains even distinction across all arcs. Even Rukia, despite her significance, undergoes periods of receded screentime that typical deuteragonists wouldn't face.
As such, the series doesn't obey standard deuteragonist patterns. However, Rukia Kuchiki occurs as the strongest nominee for this role. The debate enfolding the series' deuteragonist reminisces about the series' complex narrative structure and rich character ensemble.
Finally, the question of Bleach's deuteragonist tells the series' irregular path to character development and storytelling, where numerous characters communicate essence while one carries the strongest claim to secondary protagonist status.