A true scholar finds clues to the future in the mistakes of his past.
Ienzo is Ansem the Wise’s youngest apprentice, and the complete self of Zexion. At a young age, Ienzo lost his parents from unknown causes, and came under the care of Even and the other apprentices. After Xehanort became an apprentice to Ansem, Xehanort manipulated the other apprentices into immersing themselves into the darkness from their research on the heart, causing them to split into Heartless and Nobodies. However, Ienzo’s strong will enabled his body to become his Nobody self Zexion, one of the founding members of Organization XIII.
Following Zexion’s death and the destruction of his Heartless, Ienzo was re-completed in Radiant Garden with the other apprentices. With the threat of the real Organization XIII and another Keyblade War on the horizon, Ienzo and the remaining apprentices allied themselves with Sora and the other guardians of light with the mission of resurrecting Roxas and reconstructing his heart.
As a child, Ienzo was rather quiet, tending only to speak with those he was comfortable with. However, though he didn't speak much, similar to Aeleus, his actions spoke for him just as much.
After being re-completed along with the others, Ienzo is much more talkative, utilizing advanced vocabulary, numerous scientific phrases, and exhibits a scholar-like persona. He does have a bad habit of rambling on with his explanations. This is the complete opposite of himself as a child. This trait was still present when explaining to Sora about his findings, though he's straight to the point about some important information.
As a child during the events of Birth by Sleep, Ienzo wears a white coat, with the sleeves folded back, over a black, V-neck top with a white collar. He also wears blue pants and navy blue shoes, as opposed to the other older apprentices' black boots. He resembles his adult self, and Nobody incarnation, with a few minor changes such as shorter, lighter hair. His eyes also appear to be a lighter shade of blue.
In Kingdom Hearts 3D: Dream Drop Distance, having grown up, Ienzo's appearance matches his Nobody's exactly but wears a long white lab coat, grey vest, white collared shirt, and a purple ascot. However, his hair is much better kept and darker in appearance compared to Zexion's.
Ienzo appears in Kingdom Hearts Birth by Sleep as a minor character with no speaking lines. His parents passed away for an unknown reason, and he was given a home by Ansem in his castle, becoming the youngest of his apprentices. Disobeying the orders of his master to stay inside the castle, Ienzo was ultimately surrounded and attacked by Unversed until Ventus comes to his rescue.
Shortly after the battle, his escort Even arrives and gives Ventus both his and Ienzo's thanks, explains Ienzo's past and that he had been assigned to look after the boy, giving Ventus the directions to the Castle Garden where he would find Terra before taking Ienzo back to the castle.
As Braig is interrogating Xehanort on his state of mind, Ienzo and Ansem walk past, eating sea-salt ice cream. The pair stop and turn to see Braig and Xehanort walking away, still deep in conversation.
Between Kingdom Hearts Birth by Sleep and Kingdom Hearts 358/2 Days
In the year following the events of Birth by Sleep, Ienzo participates in Ansem's research on the darkness in hearts alongside Braig, Dilan, Even, Aeleus, and Xehanort. It would be Ienzo's persuasive influence that convinced Ansem to build a laboratory in the castle's basement to conduct the experiment on Xehanort. But though Ansem decided not to proceed further, Ienzo and the other apprentices had their own intentions and continue conducting inhumane experiments on the human heart that ultimately results in the mass production of the Heartless. Ansem quickly discovered their secret experiments, and immediately ordered them to stop. One day, after being advised to review their research data by King Mickey, Ansem found that the effects of their experiments had become irreversible before his apprentices banished him to the Realm of Darkness. However, Ienzo was tricked into believing that Ansem had gone mad and intentionally abandoned them.
In the end, Xehanort betrayed them, removing the hearts of the other apprentices, all for his plan to gather the twelve vessels he wished to house fragments of his heart to use in the creation of the true χ-blade. Though his heart turned into a weak Heartless that was eventualy destroyed, Ienzo had a strong will that allowed him to continue as a Nobody, who was given the name Zexion.
After both his Heartless and Nobody had been defeated, Ienzo is restored to his original self along with his fellow apprentices and Lea. While Aeleus had Dilan and Even rest longer to help them stabilize from their restoration, Ienzo helped him search Ansem's study while Lea searched for Braig and Isa. After Lea returns from a frutiless search, Ienzo goes on to explain that when a person is splintered into a Heartless and a Nobody, they are to be restored in the world they originally split apart. However, he then adds that there's the possibility of coming to in other worlds that are between light and darkness, such as Traverse Town. Before Ienzo can elaborate further, Lea stops him, saying he will do another sweep of the grounds.
At some point Ienzo receives a Gummiphone from Chip and Dale. He first contacts Sora and his team despite causing them trouble during his time as a Nobody. Despite the slight grudge over this matter, Ienzo was willing to give full support to Sora as both desired to bring back those who had been lost to them due to Xehanort's actions.
When Ienzo was deciphering Ansem's code in the lab, he noticed that the terminal from Twilight Town was logged in by Pence. He contacts Sora, knowing that he's the one whom attempted to log in to access to another Twilight Town. After the terminal was completed, Ienzo informs troubling news about Even's disappearance and that Aeleus and Dilan were unable to find him.
In the midst of recreating Roxas's heart, Ienzo was in a standstill of finding a body to place his heart in. While overseeing his other options, Demyx arrives through a dark corridor, carrying an empty replica vessel made by Vexen. Demyx also reveals to Ienzo his former master Ansem the Wise; Ienzo laments over the events from the past, where the others told him that Ansem had gone mad and abandoned them, but Ansem assures him that they can fix their mistakes.
After the defeat of Master Xehanort, the empty replica that once served as the vessel for Dark Riku was brought to Radiant Garden, where Ienzo, Ansem and the re-completed Even successfully worked on bringing back Naminé with it.
Shortly after Sora's disappearance, Kairi offered to go into stasis to allow Ienzo and the other scientists at Radiant Garden to search her memories for a possible clue to Sora's location. For one year, Kairi has been asleep in their care, all while Ienzo and the others search her memories, though to no avail.