Bleach tells the tale of Ichigo Kurosaki, the substitute Soul Reaper, and his adventures against the forces of evil. But Ichigo is doing more than slaying generic Hollows -- he makes quite a few rivals and enemies along the way. After the Soul Society arc ended, Ichigo's main rival became the one and only Grimmjow Jaegerjaquez.
Who was Grimmjow? He served as one of the ten Espadas under Sosuke Aizen's command, but he was always a rogue and rarely acted on Aizen's behalf. In fact, Grimmjow's personal arc depicted a desperate would-be apex predator and his pursuit of glory, with five particular scenes standing out in his tale. These moments helped define who he is.
When Grimmjow Met Five Ambitious Gillians
Before he became a powerful Arrancar, Grimmjow wandered the white sands of Hueco Mundo as an Adjuchas-class Menos Grande, desperate to sustain himself by cannibalizing other Hollows. One day, he happened upon a squad of Gillian-class Menos who challenged him to a fight. Grimmjow outmaneuvered them and took a bite out of D.Roy as a show of strength, so the five Gillians adopted the "if you can't beat them, join them" view. They knelt before this powerful Hollow and implored him to become their leader and king in this dog-eat-dog world of despair.
Grimmjow didn't expect that, but he realized that mutual cooperation could strengthen his position, so he accepted. Now with a team by his side, surely Grimmjow would become Hueco Mundo's top predator -- until Sosuke Aizen came along with a certain magical sphere.
When Grimmjow Lost An Arm & His Rank...
Grimmjow later became the 6th Espada, longing for a worthy challenge as a mighty Arrancar. One night, without permission, he gathered his five Fracciones and snuck into Karakura Town via a large Garganta Portal to challenge Ichigo and his friends to mortal combat. Grimmjow had the edge on Ichigo, but it didn't matter -- he had disobeyed Aizen, and Kaname Tosen dragged him back to Las Noches.
There, Tosen punished the disobedient Grimmjow with the loss of his left arm and humiliatingly stripped his rank. Now he was weaker and had no standing among the Espadas, further fueling his rage. This scene further established Grimmjow's position as a reckless, self-absorbed rogue and also prompted Luppi Antenor's arrival in the Bleach story. Grimmjow is far from an obedient minion. He is sheer chaos and instinct.
... And Had His Rank & Arm Restored
Grimmjow's feud with Ichigo intensified after their second duel in Karakura Town, the rogue having to retreat after Shinji Hirako, a Visored, backed up Ichigo and Rukia (in fact, Ulquiorra ordered Grimmjow's retreat). The plan to abduct Orihime was now complete, and Aizen asked her to demonstrate her powers with a handful of Arrancars and Espadas watching. Luppi was skeptical at first, but then Orihime used her incredible power to restore Grimmjow's lost arm, as well as his skin where his "6" tattoo had been.
Amazed, Grimmjow used his restored power to assassinate Luppi with impunity and regain his rank, which meant he owed Orihime a favor. He would repay this favor later by rescuing her from two bullies, and Orihime would heal Ichigo for the final fight against Grimmjow. It seemed like Grimmjow had fallen back in line, but in reality, he would use his restored power to further his own self-centered goals.
When Grimmjow Clashed With Ulquiorra Over Orihime
Bleach was quickly making clear that Grimmjow was not a loyal soldier of Aizen. He was an instinct-driven rogue, and this trend came to a head when he faced his fellow Espada Ulquiorra Cifer in combat. Grimmjow had swiped Orihime right from under Ulquiorra's nose, and when Ulquiorra demanded her back, Grimmjow attacked him. Ulquiorra responded in kind, and gained a slight edge with his superior Cero blasts.
That's when Grimmjow used a Caja Negacion, a tiny but powerful artifact, to trap Ulquiorra in a pocket dimension for a few hours. Grimmjow is now a total rogue who swore no loyalty to anyone else. In this context, Grimmjow could get his much-desired final fight with Ichigo, but could not count on any aid from Aizen or the Espadas. In fact, the other, more loyal Espadas might be hostile to Grimmjow, since he had acted against Aizen's best interests one too many times.
When Grimmjow Helped Ichigo Fight The Sternritter
Grimmjow was enraged after losing to Ichigo in their final duel, and desperately wanted to take him down despite his injuries. But Ichigo already had Nnoitora Gilga on his hands, so Grimmjow faded into the background. Despite being a sore loser, he returned in Bleach's Thousand-Year Blood War arc, grudgingly realizing that he, Ichigo and Nelliel Tu Odelschwanck were on the same side this time.
After all, Grimmjow had a distinct sense of honor and showed his more virtuous side by forgoing his feud with Ichigo to protect Hueco Mundo from Yhwach's plans. Grimmjow leaped into the fray and killed Sternritter "D" Askin Nakk Le Vaar, surviving this story arc intact. It's not clear what happened to him after that, but at least he got a chance to show his noble side. Grimmjow's not so bad after all.