He is the Arrancar #100. His number has been signified under his left eye-ball.
Cien is a chunk of reishi, formed by the threads of negaccion, which had contained haze memories and knowledge of Szayel, of Roka Palamia. Cien himself unconsciously avoided the possibility of himself being a fake and believed himself to be the real Szayel before learning the truth by Mayuri. Even after finding out the truth about himself, he had ordered Privaron Espada, Arrancar #102, Pikkaro to gather the data Szayel had abandoned for the purpose of making himself a "perfect existence." He then preys upon Roka for her threads of negaccion. After he found out the fact that he wasn't Szayel, he grew an ego and feelings unique to himself--ultimately ending up as a battle maniac, who believes that the life where one finds himself fighting to scrap off the life off of his opponent is the one that's truly worthwhile, like Zaraki Kenpachi.
-At the time of past Espada consisting of 7 core members, Szayel was its 0 Espada. Prior to him becoming an arrancar, Szayel was a VL-class Menos. Ilforte Grantz was an element Szayel had separated from himself in order to make himself a perfect existence. In a way, the existence of Ilforte Grantz and Szayel is like that of Starrk and Lilynette. In the process of separating Ilforte from his soul, Szayel faced regression from a VL-class arrancar to adjuchas-class arrancar. Due to the loss of his powers as a warrior, Szayel found himself, at one time, kicked out from Espada and became a Privaron Espada. He, afterwards, became 8th Espada by solely relying on his prowess as a scientist. Before his death as a human, Szayel was an alchemist, who had repeatedly performed grotesque human-body procedures.
When he was alive as a human, he and Ilforte were actual brothers. After their consequential deaths, Szayel preyed upon his brother's soul as his first meal as a hollow. So, to begin with, there wasn't an existence known as "Ilforte Grantz." Szayel, who had evolved into both a VL-class Menos and a Cero-Espada, in order for him to become a perfect existence, created another existence separate from himself by gathering his potential as a warrior and other parts of him with his brother's soul as a base, resulting in what is known as "Ilforte Grantz." Szayel's brother, as a human, was a leading general of a huge army. He had supplied Szayel, the alchemist, with defeated soldiers as experiments.
Although Cien's sword release is the same as with Szayel's, his combat capacity had far exceeded that of original's (Szayel's resurrected form as an Octava Espada, not his original form as a VL-class arrancar.) through the process of having Piroka to gather data abandoned by Szayel. After he had fully gathered the abandoned data, the wings on his back transform into a thing that resembles butterflies with eye-balls. The number that's been seared onto his left eye-ball also changes from "100" to "0." Cien's fully-powered form has strength comparable to that of Szayel in his prime as a Cero-Espada. Of course, the form's strength, by far, surpasses that of the current Cero Espada, Yammy.
Cien, in his final form, can shoot Gran Ray Cero's from his tentacles, mouth, and both of his hands. It is also possible for him to shoot the said technique multiple times from all of the aforementioned reiatsu-focusing points.
He is an incomplete existence, thus he is unstable in his nature. In order for him to become a stable existence, it is required for him to devour Szayel's main body, which can be currently located in Hell. So, in his current state, his body corrodes if he fights too hard. Cien Grantz stated that it would be possible for Zaraki Kenpachi of back then to be capable of cleaving Barragan's respira with a swing of his sword. He also remarked that Starrk's Colmizzo (The swords he had summoned to block Shunsui's hack-and-slash attacks.) would not reach the innards of Zaraki's body. Cien once made an attempt to crush Zaraki's internal organs through the techniques Szayel had showcased to torment Renji, and Ishida, but failed; Zaraki's reiatsu and his existence had been heightened to the point where his powers would no longer affect him, thus Zaraki's organs, at the time, were apparently too hard for even of Cien's level of power to crush.
According to Ishida Uryuu's analysis of Cien's reiatsu level, it is comparable to that of Fully-hollowified Ichigo.