There are innumerable characters in Bleach that languish at the bottom of the metaphorical barrel, mainly brought out for comic relief or vague exposition. These characters have the worst narrative luck, as they get practically zero screentime and have little to no importance in the overarching storyline.
On the other hand, there are those who play crucial roles in Bleach, whether in terms of their battle prowess, strategic victories, or incalculable supplies of reiryoku. Unfortunately, a few of these characters aren't given their chance to shine for whatever reason, and therefore never display the fullest limits of their potential.
10 Shuhei Hisagi Never Gets To Use His Bankai (If He Even Has One)
The running joke about Hisagi is that he hasn't yet reached bankai capability despite his most earnest efforts. In the concluding chapter of the manga, he tells his comrades and superiors that he actually does have a bankai, but there hasn't been a suitable time to bring it out.
Whether Hisagi is telling the truth is irrelevant, because he barely gets to fight in the Thousand Year Blood War arc despite many other Shinigami getting the chance. Considering the potential of his shikai, Kazeshini, Hisagi fighting a Quincy with the full power of his bankai would have made for an amazing battle.
9 Yachiru Kusajishi Is Turned Into A Zanpakuto Spirit
Yachiru, the Squad 11 Vice-Captain, is known for her charming naivety —she reacts to several situations as a child would, and the reason she isn't scolded for her behavior is because of her association with Zaraki Kenpachi. Yachiru finally reveals her zanpakuto, Sanpo Kenju, against Gremmy's imaginary Sternritter, but is quickly defeated (and the bones in her arm turned into cookies).
Kenpachi decides to take Gremmy on, but when he triggers his shikai, Nozarashi, for the first time, Yachiru up and vanishes, leaving just a pile of clothes behind. Only when Kenpachi is about to lose to Gerard does she reappear, encouraging him to activate his bankai.
8 Ishida Uryu Deserves More Than A Single Battle With Haschwalth
The Thousand Year Blood War arc is supposed to be Uryu's moment to shine, much like Rukia, Orihime, and Sado are central characters in their respective arcs.
He is granted the ultimate Schrift (Antithesis) by Yhwach, who proceeds to proclaim Uryu as his successor, which clearly lays the foundation for a major fight scene taking place at some point. The long-awaited battle does occur against Haschwalth, but neither the events nor the outcome of the match reach the turning point that Uryu deserves.
7 Retsu Unohana Is Forced To Be Kenpachi Zaraki's Stepping Stone
Retsu Unohana is portrayed as maternal and compassionate for most of the storyline (with certain hints dropped with respect to her origins). In the concluding arc, she is revealed as the First Kenpachi, Unohana Yachiru, plainly stating to readers that her strength is not to be underestimated.
Unfortunately, Shunsui requests her to help Kenpachi —the only opponent powerful enough to beat her in the past— learn how to commune with his zanpakuto. At the end of this process, Unohana is killed just so Kenpachi can "soar to greater heights."
6 Rukia Kuchiki Does Not Appear In The Final Battle Against Yhwach
Now, Rukia is a rather important character in Bleach, being the first to enter the narrative. However, she spends most of the Soul Society Rescue arc in Shinigami prison, and her roles in the Hueco Mundo and the Fake Karakura Town arcs are negligible, to say the least.
Rukia manages to defeat Äs Nödt with her newly matured bankai, Hakka Togame, but even this requires the moral assistance of her big brother, Byakuya. The saddest part is that Rukia is excluded in the last battle, in which Ichigo, Aizen, Renji, and Uryu team up to defeat Yhwach.
5 Jushiro Ukitake Sacrifices Himself Instead Of Fighting The Sternritter
Ukitake has been called one of the strongest Shinigami in Soul Society by none other than Yamamoto Genryusai, and he lives up to the name. Most of the time. But when Yhwach's actions begin to threaten the integrity of the three worlds, Ukitake sacrifices himself, consequently allowing Mimihagi, the Right Hand of the Soul King, to stabilize the situation.
This plan fails almost immediately when Yhwach literally engulfs both Mimihagi and the Soul King, negating Ukitake's endeavors. He could have easily been on the frontlines, instead, like his best friend, Kyoraku Shunsui.
4 Grimmjow Jaegerjaquez Returns With Great Fanfare, Only To Be Sidelined
Grimmjow's demise is not clearly indicated at the end of his fight with Ichigo, so fans naturally expected him to return again. He is vaguely foreshadowed at the start of the Thousand Year Blood War arc, making his appearance during Urahara's fight with Askin. Grimmjow is quickly brought down by the Sternritter's Deathdealing, but he hasn't been taken out of the equation just yet.
After Kannonbiraki Benihime Atarame weakens Askin a great deal, Grimmjow pulls the latter's heart out and pulverizes it. No spectacle, no fireworks, just an uncharacteristically stealthy assassination attempt.
3 Isshin Kurosaki Should Have Fought In The Thousand Year Blood War Arc
Isshin is believed to be an ordinary human by nearly every character in Bleach, even Ichigo, but audiences learn of his Shinigami roots quite early in the series. He reveals his true identity in the Fake Karakura Town arc when he takes on Aizen along with Yoruichi and Urahara.
Isshin's shikai is briefly described in a flashback scene that takes place in the Thousand Year Blood War arc, but his role is basically non-existent in the current timeline. Considering his overwhelming strength, it makes no sense to put Isshin Kurosaki on the bench when the situation turns dire.
2 Yoruichi Shihoin Doesn't Release Her Shikai, Let Alone Her Bankai
Yoruichi almost exclusively relies on her Shunko when fighting, which is honestly more than capable of taking out the hardiest opponents. Where she is unable to surpass them in strength, she does in shunpo, earning her name as Soul Society's Goddess of Flash.
However, Yoruichi does not employ her zanpakuto at all, despite being depicted with one in the Soul Society Rescue arc. One can hardly imagine the limitless power she would have wielded with her shikai, let alone her bankai.
1 Aizen Sosuke Could Have Reached Unattainable Heights With The Hogyoku
Aizen has been planning the biggest scheme of all, the destruction of the Soul King and, as a consequence, the tenuous equilibrium between worlds. His hogyoku has limitless potential, transforming him into a more powerful version several times over the course of his battle with Ichigo.
Nevertheless, Aizen's eventual defeat signifies that the hogyoku has lost its trust in his power. If Ichigo had failed, Aizen would have easily triumphed over Soul Society and, at the very least, attempted an invasion of the Royal Realm.