Szayel Appro Granz is the 8th ranked Espada in Sosuke Aizen's army of Arrancar. He is extremely smart, Planing his traps down to the finest little detail in order to capture specimens for his experiments. The remains of his Hollow mask make up the frame of his glasses.
Szayelaporro Granz was a Human alchemist employed by a particular nation hundreds of years before Aizen's defection from Soul Society, and his older brother, Yylfordt Granz, was a general in that nation's army. Yylfordt and his brother collaborated on a regular basis to provide inmates for his brother's harsh and fatal experiments. Those who died eventually turned into Hollows, who then murdered the Granz brothers as a form of vengeance.
Szayelaporro forced himself to become a Hollow and devoured his sibling, ecstatic with the discovery of an entirely new dimension of existence for him to explore. Szayelaporro eventually developed into a fearsome Vasto Lorde and joined Baraggan Louisenbairn's court, and when Baraggan was deposed by Sosuke Aizen, the rogue Shinigami transformed Szayelaporro into Espada 0 of his army.
Szayel seemed to work from the shadows. He was the original number zero Espada before giving up his powers and splitting with his brother into two different people. He did this so he could get closer to what he believed was perfection. This decision led to him being removed from the Espada. Szayel was a key factor in aiding Nnorita in the exile of Neliel. He used his research to develop a device to create a false image of Nnorita that threw Neliel off-guard. He was also the one that ripped off the masks of Neliel's Fraccion Pesche and Dondochakka. Over time he would gradually regain his status as an Espada.
Hueco Mundo Arc
Szayel defies Aizen's orders to stay put while Ichigo and friends roamed free. He dispatched the Exequias to handle the trespassers. Aizen excused his actions as Szayel claimed his actions in goodwill in eliminating a likely threat. He would ultimately decide to face Renji and Ishida in a room of his design. The pair manage to deal significant damage to the Espada. Fortunately, Szayel took getting hurt into his analysis and developed means for instant regeneration. Renji and Ishida were basically helpless as Szayel transformed in his released state.
Szayel engages Renji Abarai and Uryu Ishida in battle, during the course of which he nullifies each of their abilities and damages them enough to be in a non-combative state. Just as he loses interest in them and is about to finish them off, Mayuri Kurotsuchi arrives to help them. Being of equal scientific and analytical ability as Szayel, Mayuri is able to render all of Szayel's abilities useless. Following his next death at the hands of Mayuri Kurotsuchi, Szayel reconstituted himself inside the womb of Nemu Kurotsuchi and was puked up in the same released form he had died in albeit with Nemu's genes added to the mix now.
But what he did not realize is that using her body to reproduce a clone of his original he had inadvertently inherited the various poisons within Nemu's bloodstream, which had been placed there in the event of something like this happening, including the "Super-Human" poison, which enhances every aspect of a single being to the point where one second seems like one century to him.
With his physical body unable to compensate for his over-enhanced state, Szayel ended up effectively paralyzed. Mayuri stabbed him through his hand and heart and broke the blade off, leaving Szayel to suffer for what seems as an eternity to him, while begging for a quick death.
Quickly following his death at the hands of Mayuri Kurotsuchi, Szayel found himself in the Hell-verse or "Hell" as it is more commonly referred to alongside fellow Espada Aaroniero Arruruerie. Szayel, who was somewhat disappointed that Aaroniero's tank didn't crack, began egging him on and eventually provoked a violent response from Aaroniero who pulled out his Glotenira and was preparing to consume Szayel when the pink-haired Espada asked him why he was going to ignore the question he had posed earlier concerning where they were.
Szayel then offered an alternative solution: to stop trying to kill one another and figure out where they are together. Aaroniero didn't buy it and got into an argument concerning that just because Szayel had once been the Octava he was more powerful than Aaroniero who had been the Nouveno. Aaroniero rebutted by saying that Szayel's power was only greater when he was in his laboratory deep within Las Noches, then, just seconds before their fight could truly begin, a prisoner named Shuren appeared and told them where they were and what their current situation was.
After thanking Shuren for jogging his memory, Szayel attacked him in an attempt to learn of a way out of the Hell-verse, to which Shuren replied there was none. Shuren then explained that in order to survive the Hell-verse both he and Aaroniero would have to listen to him. It was then that Shuren's subordinates Taikon, Gunjou, and Garogai appeared, all of the hideous and monstrously powerful human-like beings. Shuren then demanded that the three instruct their "guests" on how to be satisfied with their current situation. From behind Szayel, a furious post-release Aaroniero appeared and attempted to devour the eerily smiling and incredibly overweight Taikon only to be obliterated by his spiritual pressure in the process.
With everyone distracted, Szayel released his Zanpakutou and attempted to perform a Gran Rey Cero on Shuren only to be destroyed by his own power when Shuren's reiatsu caused the Cero to detonate in Szayel's hands, obliterating him in the process as well.
Spirits are Forever with You
During the course of the novel Spirits are Forever with You, a clone of Szayel known as Cien Grantz appears and battles the Shinigami of the Soul Society. He's revealed to be the 100th Arrancar convinced to be the real Espada, but he later learns the truth of his origins by Mayuri. This form of the character possesses a Segunda Etapa and the 100 tattoos under his left eye-ball. The novel takes place in the time skip between the Arrancar Saga and the Tale of the Lost Agent, but its canonicity (as well as Cien) is debated.
Szayel has several methods of analyzing his opponent's abilities, such as studying the damage they cause to their past opponents. Once he has enough information on his opponent's skills, Szayel can nullify their abilities, making him almost immune to their attacks.
In addition to the immunities gained from studying his opponents, Szayel created his own Fracción, his personal entourage of Arrancar. He has personally modified each of them so that when he needs to, he can heal any of his wounds by eating one of them.
Szayel's Zanpakutou is Fornicaras. Szayel releases his Zanpakuto by swallowing it. His body then swells up to enormous proportions and then explodes, revealing Szayel's released form. In his released form, Szayel has a pair of wing-shaped appendages from his back while his lower body transforms into a flock of long tentacles. Blood-looking liquid drips from his clothes and fingertips. In Szayel's released state, When droplets of the liquid dripping from his body touch an opponent, a copy of them is created that uses their own abilities against them.
By wrapping an opponent in his wing-like appendages, Szayel can create voodoo dolls of them. Any damage done to the dolls' internal organs is done to the original. His final ability, called "Gabriel", allows him to be reborn if he is injured beyond repair by impregnating an enemy. He then eats his way out of their body, killing them in the process.