Synopsis
Gin's very first appearance is alongside Captain Kenpachi Zaraki when the two catch and converse with Captain Byakuya Kuchiki after the latter delivers Rukia Kuchiki's sentence. He then appears at the gates leading into the Seireitei, following Jidanbō's defeat by Ichigo Kurosaki. He easily repels the two without killing them, simultaneously closing the gate which Jidanbō was holding open (it is later revealed by Aizen that he ordered Gin to do so). The other captains question his failure to kill opponents whom he should have had no trouble killing, but a second intrusion by Ichigo's group, followed by the supposed death of Aizen, leaves them with bigger concerns. Aizen, having faked his death, has Gin deliberately cast suspicion on himself, thereby drawing attention away from Aizen's activities. After Aizen's plot is revealed and they retreat to Hueco Mundo, Gin bids Rangiku Matsumoto farewell and apologizes to her in one of the rare moments in which he doesn't smile.
In Hueco Mundo, Gin jokingly chides Aizen over his habit of playing around with his subordinates. When Ichigo's group breaks into Las Noches, he observes them, even altering the position of the halls when no one is around. When Ulquiorra Cifer catches him in the act, Gin denies changing anything, saying that he wouldn't do that to "those kids" and that he does not like "sad stories". He later appears alongside Aizen and Tosen in Karakura Town, commenting on Kira's sudden change in emotion to Tōsen and remarks that he is glad Kira is 'doing well'. After the arrival of the Vizards, Gin cuts Hiyori Sarugaki in half at the waist with his shikai when Hiyori gives into one of Aizen's taunts. He is later seen observing Sosuke Aizen engaging the Soul Reapers, commenting on Aizen's true power, before personally fighting Ichigo with his Bankai. Easily overpowering him, Gin gives Ichigo several attempts to run, mocking his lack of determination, before traveling to the Soul Society with Aizen in order to destroy it.
In Karakura town, Gin is confronted briefly by Rangiku who, after questioning his motives, is immobilized by Gin's kidō as she is "in the way". He then returns to Aizen's side, claiming that he had killed her. Once Aizen says that he will kill Ichigo's human friends before destroying Karakura town, Gin restrains Aizen's sword, saying that he will kill the humans himself. He then activates his zanpakutō and pierces through Aizen, explaining that he could stop Aizen's Kyōka Suigetsu by touching his sword before the hypnosis starts. He then explains that he lied about his Bankai earlier and that its true power is to produce a poison capable of eroding human cells.
As Aizen's body dissolves, Gin removes the Hogyōku from Aizen's chest, thinking that without it, Aizen cannot survive. He is proven wrong when Aizen revives and assumes a new form, stating that whether the Hogyōku is inside of him or not is irrelevant, as it has already chosen him. Aizen then grievously injures Gin in front of Ichigo's friends. Rangiku awakens and rushes to his side, and as he lays in Rangiku's arms, Gin silently apologizes for being unable to return what Aizen had taken from Rangiku as a child. When Ichigo arrives and Gin sees his determination to stop Aizen, he decides to entrust Ichigo with his goal.
Read more about this topic: Gin Ichimaru