Ienzo was one of the six apprentices of Ansem the Wise who made his debut in Kingdom Hearts Birth by Sleep and Kingdom Hearts Birth by Sleep Final Mix. Ienzo is the original persona of Zexion; his existence continued on as Zexion after his fellow apprentice, Xehanort, led him to his gradual descent into darkness. Ienzo is also mentioned in the Secret Ansem Reports found in Kingdom Hearts II. After being re-completed, he became an important ally of Sora.
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.
In the end, Xehanort betrayed them, removing the hearts of Braig, Dilan, Even, Aeleus and Ienzo himself, all for his plan to gather the 12 vessels he wished to house fragments of his heart to use in the creation of the true X-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 he 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 completing Roxas' heart, Ienzo was in a standstill of finding a body to place a heart in. It was then Demyx arrives through the Corridor of Darkness to the inside of the lab, carrying a vessel made by Vexen . He reunites with his former master Ansem the Wise, feeling depressed over the previous events but Ansem assures him that they can fix their mistakes. He and the other apprentices work on transferring Roxas' heart into the vessel, completing the process just in time for the later to head to the Keyblade Graveyard to save his friends.