Grantz Szayel Aporro
Szayel is the scientist of the Espada, developing various spiritual equipment for use by the arrancar. Using various methods, one of which involves implanting a subject with rokureichū (a data-gathering device), he is able to analyze the spiritual power of an opponent through those that person has defeated. He is also adept at creating devices that can manipulate and suppress spiritual power, as evidenced in his battle with Renji Abarai. Much like Mayuri Kurotsuchi, he is quite sadistic and seems to view others as little more than research material.
Szayel is portrayed as a stereotypical intellectual. He is quite fastidious, often having a plan that he follows down to every detail and throws tantrums when they are disrupted. Szayel is also snobbish and condescending, openly or otherwise, to both allies and enemies, especially to entities who aren't hollows. This behavior also extends to his motive for his allegiance to Aizen, as he hopes Aizen will do away with the non-hollows once he becomes the king of Soul Society. Despite his attitude, Szayel is mockingly considerate enough to quiet his followers and remain quiet himself so his opponent can say something.
Szayel sometimes acts without Aizen's approval, and while he is very polite to Aizen in person, gestures he makes when his back is turned suggest that he resents him. Like the rest of the Espada, he possesses numerous Fracción, though his are hollows he personally modified and had made into arrancar by Aizen. Two of his notable subordinates are Lumina and Verona, both of whom are round arrancar with bare feet and sparse hair, though Lumina was eaten to heal her master. Szayel seems to show the same lack of care for his creations as Mayuri, using them as fodder in battle. When Verona speaks up against this after Lumina is devoured, Szayel quiets her by saying he'd just "make a new one" later.
Szayel first appears when Aizen gathers the Espada to inform them that Hueco Mundo has been invaded by Ichigo Kurosaki, Uryū Ishida, and Yasutora Sado, and, despite the order to stand by idly, Szayel orders out the Exequias, though Aizen doesn't punish him. He later faces Renji, Uryū, Pesche Guatiche and Dondochakka Bilstin, during which his resourcefulness and analyses of his opponents allow him to come out on top. Szayel finally meets his match with the equally cunning Mayuri Kurotsuchi. Kurotsuchi, having observed Szayel's abilities through organism implanted in Uryū's body, counters by replacing his organs with artificial ones, then gains the upper hand by releasing his bankai and having it consume the Espada. Szayel, however, is reborn through Nemu Kurotsuchi via another of his abilities, which allows him to implant himself into his enemies and reform even after death. But unknown to Szayel, Kurotcuchi had earlier created failsafes in the event his daughter's body was ever consumed or invaded. Among these was a drug that vastly augmented Szayel's reaction time and perceptions, to the point where one second seems like a century to him. With his physical body unable to compensate for his over-enhanced mental state, Szayel is effectively paralysed. Kurotsuchi then proceeds to stab him, informing him that, as a result of the drug, the agony of every injury will seem to drag on for a hundred years.