Kingdom Hearts Birth by Sleep
Ienzo appears in Kingdom Hearts Birth by Sleep as a minor character with no speaking lines. His parents passed away for an unknown reason. Disobeying the orders of his master to stay inside the castle, Ienzo was ultimately surrounded and attacked by Unversed until Ventus came to his rescue. Shortly after the battle, his escort Even arrived and gave Ventus both his and Ienzo's thanks, explained Ienzo's past and that he had been assigned to take care of him since the death of his parents, gave Ventus the directions to the Outer Gardens where he would find Terra, and returned to the castle with Ienzo.
 Blank Points
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
Ienzo was still one of six apprentices of Ansem the Wise, the ruler and scientist of Radiant Garden, alongside Braig, Dilan, Even, Aeleus, and Xehanort. Worried the Darkness in people's hearts would eventually consume his world, Ansem began performing tests on Xehanort's heart. Eventually, at Ienzo's persuasive influence, he created a laboratory in his castle's basement for such experimentation. However, Ienzo and the other apprentices had their own intentions for the laboratory, and began conducting inhumane experiments on the human heart and the darkness within it. Such experimentation led to the creation 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.
Eventually, Ienzo and the other five apprentices dabbled too deeply into darkness through their continued experiments, and Xehanort's memories are reawakened. After Xehanort plunges Even and Ienzo into darkness, Braig stumbles upon the scene but is also taken out. Ienzo, along with the others, was consumed by the darkness, producing a Heartless. However, because he and the other apprentices each had strong wills, Ienzo became a Nobody that was given the name "Zexion". The six Nobodies formed Organization XIII, and banished Ansem the Wise to the Realm of Darkness.
 Kingdom Hearts 3D: Dream Drop Distance
Ienzo is restored to human form, now as an adult, following the destruction of his Heartless and Zexion. He awakens in Hollow Bastion along with Dilan, Aeleus, Even, and Lea, and they soon discover that Braig and Isa are missing. Ienzo explains to Lea about how a person is meant to return to human form in the same world he or she lost his or her heart, which prompts Lea to go out to search for Braig and Isa. Interestingly, Ienzo does not appear bitter towards Lea for his betrayal during their time in the Organization.
Even as a child, Ienzo wore the same uniform as Xehanort and Even, the other researchers apprenticed to Ansem. He wears a white labcoat over a black, V-neck top with a white collar. Likely due to his size, Ienzo folds the sleeves of his lab coat back, and even then his hands are partially covered. He also wears blue pants and navy blue shoes. As an adult, he dons a purple ascot, as well as the knee-high silver-trimmed boots worn by the other apprentices.
As a child, Ienzo was quiet, yet observant and curious. It appears that he spent most of his time with Even, sharing with him his love for knowledge. He also seemed to have a good relationship with Ansem, as they are seen sharing sea salt ice cream together while Ienzo holds his hand. It was also due to Ienzo's persuasion that Ansem built his laboratory, implying that he valued Ienzo's opinions despite his young age.
As an adult, Ienzo appears much more talkative and scholarly, jabbering while explaining to Lea how Nobodies are reborn. He also appears to be rather cheerful and does not seem to hold any grudges against Lea for betraying him and convincing the Riku Replica to absorb his life force as a Nobody.
 Notes and references