Tier Harribel is an Arrancar who served under Sosuke Aizen, the former 5th division captain of the Gotei 13 and the main villain of the Arrancar Arc. Appearance-wise, she looks no different from a regular human, though her incredible abilities and fighting prowess make her stand out.
Tier was a member of Aizen's elite Arrancar squad called Espada. She bears the number "3" among the ten Espada. So as one of the strongest Espada, it goes without saying that she played a major role in the Arrancar Arc. That being said, the story somehow moves along without ever mentioning the fate of Tier Harribel. So what happened to her at the end of the series?
Arrancar Arc
Before talking about her current situation, it would be best to first look back at Tier's journey thus far. As mentioned earlier, Tier Harribel appeared mostly in the Arrancar Arc. That being said, her screen time was somewhat limited compared to other Arrancar, such as No. 6 "Grimmjow", No. 4 "Ulquiorra", or even the former No. 3 "Nelliel".
For the most part, she stays in the background and only has minimal dialogue here and there. The fact that she rarely talks makes it even harder to notice her existence. Then the final fights at the fake Karakura town happen, which was pretty much her biggest scene in the entire series. It's there that Tier fought the captain of the 10th Division, Toushirou Hitsugaya.
She unleashed her full power and gave Hitsugaya a very hard time. It took a captain and several Vizards in order to put enough pressure on her. Unfortunately, after the No. 1 "Coyote Starrk" and No. 2 "Barragan Louisenbairn" lost their respective battles, Aizen decided to strike Tier because she was no longer necessary to him. That was the last time we saw Tier Harribel in the Arrancar Arc.
The Thousand-Year Blood War Arc
After the event of the Arrancar Arc where numerous powerful Arrancar died, including the king of Hueco Mundo, Barragan, there was a power vacuum in Hueco Mundo. As one of the few high-ranking Arrancar who survived the battle, Tier Harribel assumed the position as the new ruler of Hueco Mundo. She received help from Nelliel and Grimmjow, along with several other relatively powerful Arrancar. That’s why for a while, there was finally peace and order in Hueco Mundo.
Unfortunately that order didn't last long. The Quincy's Wandenreich invaded Hueco Mundo under the orders of their leader, Yhwach. Tier and other Arrancar tried their best to fight the invaders, but alas, their struggle was futile against the almighty Yhwach.
However, rather than killing Tier, Yhwach decided to capture and imprison her. Knowing their queen was held hostage, the Arrancar had no other choice but to obey Yhwach and the Quincys. Some of the Arrancar that were deemed to be useful were spared and forced to do various menial works, while the rest was massacred. In this arc, Tier Harribel was last seen defeated and in very bad shape.
Bleach: Can't Fear Your Own World
In 2017, Ryota Narita, under the supervision of Tite Kubo, published a series of novels called Bleach: Can't Fear Your Own World. The story takes place not long after the events of the Thousand-Year Blood War Arc. It offers further explanation, some closure, and also a new crisis in the world of Bleach.
One of the characters that gave fans much-needed closure was Tier Harribel's. As mentioned before, Tier Harribel was still imprisoned inside Yhwach’s Wandenreich during the Thousand-Year Blood War arc. Near the end of the last arc, Yhwach created a palace called Wahrwelt by combining the Soul Society's royal palace with Quincy's Wandenreich. So Tier Harribel is technically in soul society after this.
Fast-forward to the start of the events in the novel, while the shinigami are still repairing the heavily damaged soul society and the royal palace, two members of the Espada breakthrough the Wahrwelt in order to rescue Tier Harribel. They are the former No. 3 Nelliel and No. 6 Grimmjow. They make contact with several high-ranking Shinigami officers, but both parties can settle their differences pretty quickly. After all, both sides suffered tremendous amounts of damage from their respective battles with the Quincys. So now, rather than fighting, it is more productive to mind their own business instead.
After that, all three Espada make their way back to Hueco Mundo. Just like after the Arrancar Arc where several high ranking Arrancar died and left the Hueco Mundo into a state of disarray, the same thing happens now thanks to the Quincy. So once again, Tier Harribel is crowned the queen of Arrancar and rules over the vast domain of Hollow with Nelliel and Grimmjow at her side.
There's a lot of information about the world of Bleach and the character who live inside it within the novel Bleach: Can't Fear Your Own World. After all, this novel includes characters from numerous factions in Bleach, namely Shinigami, Arrancar, Quincy, and even Fullbringers. It ties up some loose plot points and as mentioned before, provides some much-needed closure to some characters from the original series. Not to mention the new storyline is also very interesting by itself. Fans of Bleach manga and anime should definitely read this novel.